he absolutely attests that T4b includes the continuous present
Yes, I assumed he did, because both Pooley and Coulter–independently of each other, but they are both former students–make the same claim about the present perfect continuous, which is not to my knowledge found in any other non-Reginaldian textbook.
I have only seen it used in the construction you show in A&G with the use of words like “iam”. I cannot attest to any historical usage from my own reading.
I’d suggest it’s because it doesn’t exist. As above, I’ve never seen it in a single grammar outside of those of Foster and his students.
My guess is that it’s classical but early, but that’s not something I can confirm from anything I have at hand.
I don’t think you’ll be able to confirm that T4b [=the present perfect tense] includes, in Latin, the perfect continuous tense. I cannot have the confidence that Foster must be right, and that therefore it must have existed somewhere, sometime, especially since he was, without doubt, absolutely wrong on the sequence of tenses, which I’ve delved into and re-examined since my initial post here, as I had assumed that Foster was a Latin expert and I rather took his Ossa at face value when I read it five years ago or so. I should have been more discerning, especially since there was other quirky stuff in the book (off the top of my head, his insistence that the gerundive wasn’t a future passive participle denoting necessity but an adjectival form of the gerund–whatever that means).
The idea that nescimus quid fecerint can be represented with the translation: “we do not know what they will have done” (p. 314 where he has “have thought” instead of “have done,” presumably by mistake), is wrong, seriously wrong. Either that or he has the truth and every other textbook and grammar have the sequence of tenses wrong.
He has misread, I think, Gildersleeve and Lodge’s description of the sequence of tenses. I also think the same misreading is likely the source of his error concerning the present perfect continuous tense as well, as this tense is given as one of the possible translations of the perfect subjunctive in primary sequence (since the perfect subjunctive can represent any tense, perfect or continuous, that takes place prior to the main verb). You won’t find any grammar, anywhere, by anyone, who would support Foster’s sequence of tenses (given in a diagram on page 309).
The perfect subjunctive in primary sequence can only represent a tense anterior to
the verb in primary sequence. It can’t represent a “future completed” (p. 309) taking place after that verb.
Likewise, the pluperfect subjunctive in secondary sequence can only represent a tense prior to that of the main verb, not one that is “future completed” to it.
So it’s not hard for me to believe his grasp of the Latin tense system was eccentric, as he was. He always comes across as dismissive of everyone else, of thinking he knows more and knows better. Well, maybe he was the one who was mistaken.