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.