If you don't mind cheating, at the beginning of the game use the teleport cheat and teleport out of Irenicus's dungeon. Imoen will not be kidnapped. Be warned I haven't tried it up to the asylum to see if it glitches there or you end up with 2 Imoens or something. But there's a solution if you like Imoen but think she's too low level when you rescue her.