I, on the other hand, have no particular problem with assuming "time of join" on a character is slightly later than it is in-game, because otherwise some characters are really hard to price fairly, but Dungeon diversity is good, and F1-F3 are much tougher than they used to be. So Chris w/ True Water Rune, Meru w/ Dragoon, Kurando w/ Tsukiyomi, etc. are all fine by me... although I'd probably assume that Meru is frozen at time-of-getting Dragoon for the early floors. A certain amount of "punish characters for slow starts" is fine, seeing characters grow is part of the appeal of the Dungeon, but not when it gets too harsh / swingy. (And Marcus or even Athos will not be enough to guarantee early passes any more.)
That said, I'd be fine with a slight hack if needed (e.g. FF6 Celes, who gets Espers way sooner than she "should," and she needs to), except I don't really think Meru needs the hack? She's a 0.5 pick. If she got solid MT healing on F2, she seriously puts a lot of other 0.5 healers in trouble. Furthermore, for me at least, LoD characters translate well to the Dungeon in that for fights other than Fight 1 / fights after a full reset, they can usually stall and stock up 5 turns of dragoon meter. So Meru is actually a lot tankier than she looks in 4 battles out of 5 because she can Dragoon transform on her speedy first turn 1. So I'd be fine with letting the rest of her skillset come at in-game times rather than sped-up times.