The most stuck sketch I tried it with was one 
stuck to almost all the paint, on a cel that is 
pretty filled up with content. My attempt to 
remove the sketch was unsuccessful, but I did not 
leave the cel in the freezer overnight. For ones 
that were not as strongly attached to the cel, 
the damage ranged from nothing to slight, where I 
used a knife to finish the job of peeling the 
sketch off. Again, I did not leave the cel in the 
freezer for a very long time.

IMO, give this method a try, freezing the cel and 
sketch for 15-20 minutes and see if the condition 
ameliorates. I don't think that will harm the 
black paint, and if the sketch comes off easier 
afterwards, all the better. If not, you're still 
at square one with nothing lost.

---Trebuchet