Athos already is an exception, so no need to change anything there. I don't really see any need to change much else... looking through the list of Dungeon picks... the only potential split is whether to interpret "scale at join time" hardcore for things that are blatantly level-based, like Suikoden magic charges or FF6 MP. Nobody should ever be caring about the presence or lack thereof of legal equipment to equip when a character literally isn't in the party; rewarding or punishing based on that is crazy. However, it is very easy to, say, recognize that Floor 1 Strago has MP issues, or that F1 Sasarai has fewer magic charges. I'd be fine with either interp in such circumstances; Sasarai would like a scale-to-join-time interp that gives him a bunch of magic charges on F1-F5, but it's also very easy to not give him that benefit while scaling his general stats to join time.
* Billy doesn't join THAT late and has the same problem of "no equipment for him exists earlier so buh."
* Rosa & Rydia have long periods outside the party, so I guess a crazy voter could see Rydia as some kind of puny on Floors 3 &4, but meh to that, she should magically jump from either her time just before she leaves in Fabul to her rejoin time. (And she can probably equip Porom / Palom / Tellah's stuff anyway.)
* Magus has the same "I don't even know how to interpt him like Orlandu" issue due to lack of equipment. Hypothetical earlygame Ayla is probably really strong since she has a midgame weapon in her fists, but the only people who know for sure are those who do CT LLGs, which shouldn't be required to be able to interpret the Dungeon correctly.
* Elincia is easily interpreted for weapon availability at least.
* Sasarai, Gilder, & Strago are discussed above.
* And I stopped reading at 1.5.