I think I've got a way to do it. I couldn't find anything relevant in the LSLib database output, but the
EasyCheat mod is capable of summoning origin characters into your party - with lots of warnings about the risk of breaking the story if you continue with them! But yeah - should be OK for posing them.
Only trouble is, I tried it in my evil game. So Jaheira spawned as a naked corpse; I can revive her and then she happily walks around Baldur's Gate in the nude - I assume this is a druid thing. Laezel spawns in incapacitated and can't be revived or helped, because in this game right now she's a bit tied up with Orin. Kind of interesting to see that, actually. So, I guess I should get back to the actual plot! But I'm pleased I've found a way to do it, and I thought I'd write it here in case anyone else wants to do something similar
