Can I just say first how delighted I am to find this group of people willing to join in discussions such as these, and offering such interesting suggestions!
Thank you, Paul Derouda, for supplying some more text from Sens’s commentary. If ever I manage to lay my hand on the complete text, which may yet hold more relevant information, I will be sure to add it here (unfortunately the book’s price is rather prohibitive, and at my university the book is supposedly available, but in reality nowhere to be found; the database lists it as a specific order by/for one particular professor, who probably keeps it in his office or at home).
Jeidsath, may I ask what you mean by “the ἔπι (not ἐπὶ) causing a shift from nominative to accusative?" I’m not really clear on this. You're saying that the Pleiads should be the subject, but the preposition (with accent as if in anastrophe?) puts it in the accusative anyway?
Also, I looked up the line you quote (οὓς Πλειάδα ἐκάλεσαν διὰ τὸ λαμπροὺς εἶναι ἐν τῇ τραγικῇ ὡς τὰ ἄστρα τῆς Πλειάδος) and found it to be from Georgius Choeroboscus’ commentary on Hephaestio of Alexandria. Apparently it refers to a circle of tragic poets in the day of Ptolemy Philadelphus being nicknamed the ‘Pleiad’, either because they were so brilliant, as the grammarian himself suggests, or maybe because there were seven of them, as Evina Sistakou suggests in her book ‘Tragic Failures’ (which I found on Google Books by looking up the Greek quote). Either way, Πλειάδα seems to me to be acc.sg.f. there; I don’t think it ever occurs as a n.pl.
Paul Derouda, I have to say I don't see much similarity between Od. 1.421f and the epigram; more so with the Sappho fragment (fr. 52 Bergk = fr. 74 Reinach) jeidsath mentioned, which had also crossed my mind. But of course it can’t be exluded that a construction with μένω was used here before some monk dozed off on the job.
So anyway, I dropped by the university today, and found some more interesting tidbits.
In Hugo Stadtmueller's Teubner edition (in which this epigram is 5.188 rather than 189) I found some suggestions in the critical apparatus: one Mähly (probably Jacob Achilles Mähly, 19th century Swiss scholar) apparently suggested reading either Νὺξ μακρὴ καὶ χεῖμα μέσον περὶ Πλειάδα δῦσαν or Νὺξ μακρὴ καῖ χεῖμα μέσον, Πλειὰς δ’ ἐπέδυνε. Pretty clever suggestions, I think, though of course we can't prove either of them is correct. In both of them, he changes μεσήν to μεσόν, to make it go with χεῖμα. In the first he takes both nouns together as a double subject, and puts the verb in the plural. In the second he supposes an ellipsis of ἐστίν, and makes the Pleiads subject of the next verb, along the lines of what several people here suggested.
The editor of this Teubner edition adds, in the critical apparatus, regarding δύνει: ‘fort[asse] θύνει vel θύει: cf. Hes. Th. 621’.
He made a small mistake there in the reference, which ought to point not at the Theogony, but line 621 of the Works and Days, where we find:
Εἰ δέ σε ναυτιλίης δυσπεμφέλου ἵμερος αἱρεῖ·
εὖτ’ ἂν Πληϊάδες σθένος ὄβριμον Ὠρίωνος
φεύγουσαι πίπτωσιν ἐς ἠεροειδέα πόντον,
δὴ τότε παντοίων ἀνέμων θυίουσιν ἀῆται· (618-21)
Glenn Most’s Loeb edition prints θυίουσιν here, but the manuscripts have θύουσιν, and I’ve often seen this verb, meaning ‘rage’, spelled θύω, even though it has nothing to do with that other θύω ‘sacrifice’. A scribal error where δύ(ν)ω is confused with θύ(ι)ω doesn't seem far-fetched at all.
By the way, Most translates these lines:
‘But if desire for storm-tossed seafaring seize you: when the Pleaiades, fleeing Orion’s mighty strength, fall into the murky sea, at that time* blasts of all sorts of winds rage’ (his note: * in November).
Despite misgivings, Stadtmueller’s Teubner does not obelize the second half of the first verse.
Andrew Gow and Denys Page, in their 1965 edition and commentary of the Anthology, already did obelize, however. And in the commentary they discuss the problematic half line at some length, they too calling it ‘a very obscure phrase’. I was going to quote only the most interesting bits, but ended up copying the entire entry, so if anyone is still hanging in there, here it is (with minor additions and notes by me for clarity and ease of reading and simply because I like to follow up on these leads far beyond the point where they cease to be of any interest to anyone):
“[Friedrich] Jacobs supposed the subject of the verb to be νύξ, nox ad occasum vergit, and added Vergilias in media cursus sui parte collocatas significare videtur. Voluit poëta simul noctis horam et anni tempus indicare(*). But apart from the difficulty of going back to νύξ for a subject, this interpretation of μέσην is hardly credible, and for νὺξ δύνει the only parallel we can supply is the converse ὀρώρει δ’ οὐρανόθεν νύξ(**) (Od. 5.294, al[ibi]; see [A.S.F.] Gow on Theocr[itus] 13.10(***)). [Alfons] Hecker wrote μέσον, and translated media hiems condit (ἐπιδύνει) Pleiadem(****), referring to the setting of the constellation in late autumn (see below), but ἐπιδύνω is not elsewhere transitive, and the setting of the Pleiads regularly marks the beginning, not the end, of winter. [Jean François] Boissonade supposed the subject to be ἥλιος, but we are no more able than he to supply a parallel for the ellipse of this noun.
Boissonade cited Hygin[us] [De] Astr[onomia] 2.21(*****) and might have added other passages from minor Greek calendar-makers (see [John] Lydus de Ostentis pp. 350, 357 Wachsmuth(******)), who also record the matutinal setting of the Pleiads at the end of October or beginning of November as the beginning of winter (see [William] Smith Dict[ionary of Greek and Roman] Ant[iquities] 1.227), to which, if νύξ can be the subject of δύνει, the phrase might refer. Ἐπί c[um] acc[usativo] when temporal should mean towards or about, and ἐπὶ μέσην Πλειάδα might mean about the time when the middle of the constellation is setting, midway in the setting of the Pleiads, but the Pleiads form so small a group of stars that one would not expect such a distinction to be drawn.
We should expect the phrase to denote the time of year, but do not see how such sense can be reasonably extracted from it without drastic alteration. If the author had written Κύων δ’ ἐπὶ Πλειάδι δύνει, the (cosmical or matutinal) setting of Sirius is now following that of the Pleiads, i.e. we have passed from the beginning to the end of November (see Smith Dict. Ant. 1.230), the sense would be very satisfactory; but the suggestion has no transcriptional probability, and we mention it only exempli gratia in default of a better.”
(*) 'Night draws to its setting' (i.e. begins to set); and 'This seems to mean that the Pleiads are placed in the middle of their course. The poet wanted to indicate the hour of the night and the time of year at the same time.' (my translations)
(**) Also Od. 9.69 and 12.315 (but nowhere in the Iliad). Translated in the current Loeb edition by Augustus Taber Murray, revised by George E. Dimock: 'and down from heaven night came rushing' (with ὄρωρα from ὄρνυμαι).
(***) Theocritus 13.10: χωρὶς δ' οὐδέποκ' ἦς, οὔτ' εἰ μέσον ἆμαρ ὄροιτο, translated in Neil Hopkinson's new Loeb edition: 'He was never apart from him - neither as midday came on,' (etc.). Andrew Gow in his commentary discusses the use of ὄρνυμαι for the coming of nightfall and daybreak, and strangely also noon, but none of this is relevant to the issue at hand.
(****) The middle of winter hides the Pleiads. (my translation)
(*****) Pseudo-Hyginus, last line of Poeticon Astronomicon 2.21: Eas stellas nostri Vergilias appellaverunt quod post ver exoriuntur et hae quidem ampliorem ceteris habent honorem, quod in earum signo exoriente aestas significatur, occidente autem hiems ostenditur, quod aliis non est traditum signis. (Translation by Mary Grant: 'Our writers call these stars Vergiliae, because they rise after spring. They have still greater honour than the others, too, because their rising is a sign of summer, their setting of winter - a thing is not true of the other constellations.')
(******) Curtius Wachsmuth's 1897 edition of John the Lydian's De Ostentis and excerpts from other authors about the calendar is full of unexciting lines like Clodius Tuscus's 'τῇ πρὸ θ’ καλενδῶν δύονται αἱ πλειάδες' ('The Pleiads set on the 24th of October') and Columella's ‘VI Id. s[upra] s[criptas] Vergiliae mane occidunt; significat tempestatem; hiemat' (’On the 8th of that month (i.e. November) the Pleiads set at dawn; this means storm; it's wintry').