I think they could probably leverage some of their existing programming to prevent class changes to classes under a certain ilvl, because of something I've noticed. I have a separate PVP vs PVE loadout for my casters, with the PVP loadout being lower ilvl than the PVE loadout but with pretty much all Crit/Det for secondaries (since after the level sync down to i80, some of my i80/i90 pieces are better for PVP than some of my i130 ones once they've been synch'd down).

Anyway, if I register as BLM/SMN and I have my PVE set equipped, and the commence window pops, if I try to change to my PVP set at that point, I get a message that I'm unable to change gear at that time. That message only pops when I'm trying to go from a higher ilvl to a lower one, and not if swapping from a lower ilvl one to a higher one, so there's some kind of check programmed to catch that. As soon as the instance starts, I can swap to my lower ilvl (but still above i80) PVP set, but just as they can prevent gear swaps to a lower ilvl after hitting commence, seems like they should be able to prevent a class swap to an ilvl < i80 inside the instance. It wouldn't prevent people from wearing totally useless accessories to cheat the ilvl but that's just a small subset of the problem and it seems worth fixing.