In this recurrent platonic phrase οίον το τοιούτον λέγω, There is an ellipse of εστί, and, λεγω is used in the future sense. Right?
Do you mean οἷον τὸ τοιόνδε, instead of τοιούτον?
It occurs in Phae. 65b and Rep. 353d. Other than that I can’t say whether it is a specifically Platonic idiom or not. It is, however, quite a useful way of elaborating on a presented argument.
As for an ellipse, no. οἷον operates adverbially as an accusative of respect (normally translated as “for example”) to which τὸ τοιόνδε lies in apposition. For the function of this demonstrative see Smyth 333 and LSJ s.v. τοιόσδε. It is generally translated “as follows,” i.e. “the following thing” as LSJ recounts (albeit with a caveat), and from this one might (in context) read (or translate) λέγω as future.
In English I would say “I shall provide the following example.” More literally “for example I say this thing that follows.” Hence why in both of the texts mentioned above (in my editions at least) this phrase is followed by a colon, thus:
οἷον τὸ τοιόνδε λέγω·…
Thanks a lot.