I can vouch for the freezer trick to work on the 
few cases I tried. This method works better for 
sketches that are not very stuck to the paint, 
but maybe it'll work fine in your case because 
there is only the black paint.

For my freezer (a dorm fridge), I have to wait 15-
20 minutes before I can peel the sketch off 
easily, depending on the paint and how stuck the 
paper is. You can feel the paper come right off 
the cel when the method is working at its best. 
If it doesn't work the first time, refreeze it 
for another 10 minutes before trying again. If 
you feel the paper won't give anymore, it's up to 
you whether you want to leave the cel freezing 
overnight (probably best to remember to wake up a 
little earlier and try to peel off the sketch 
again before heading to work) or decide that it 
is time to try a different tactic.

---Trebuchet