>>538008460
Earliest MegaTen yaoi I have seen is the Persona 1 doujinshi by the P1 mangaka (and it opens with this funny ass page).
Then P2 directly pandered to fujos, so you know it was a thing before then.

But MegaTen in general, I did not see much yaoi or hentai made before the 2000s. I saw one raunchy Tamaki/Reiko/Jack Frost porn comic in those ancient doujins with circles full of authors sharing the same space just to shitpost about the games and anime they liked at the time.
It seems not many people were interested in drawing SMT porn and that general audience doujins and funny gag manga were more popular with fans of the time.