I just finished using a lighter to separate some plastic from plastic.
Burned it gooooooood!!!!
*Ahem* But that's only really an option if you can stand for some damage of the piece. I could get away with it because once the model is assembled you won't see the effect (which wasn't too pretty).
For what you're aiming to do, I wouldn't recommend it; but some people could use it in other projects. I'll say it again though - it's not for those who want to keep the plastic in good condition. Desperate times, desperate measures, yadda yadda.