In the past, I've never dealt with backlighting, 
but one of the main questions which pops to mind 
is... how much space will you have between the 
front of the frame, the cel, the back of the 
frame, the light source, and the wall?

It would seem that you would need a very flat way 
of lighting it. One of the nice things about 
distance lighting is that the light is 
diffused... it's not all concentrated in one 
spot. You'd have to find a way to diffuse the 
backlighting in a very short space... otherwise, 
it might be kind of like putting a flashlight 
under the palm of your hand. ^_^ You can "see 
through" a circle of your hand as wide as the 
flashlight is, and then everything else is just 
dark. So not only would it have to be flat (to 
fit up against the wall), but it would have to be 
wide... or perhaps you'd have multiple light 
sources in back.

Another issue you might look at is the heat 
generated. The lower the "color" of the light, 
the less heat it generates. Incandescent light 
bulbs generate red light, bright light, hot 
light. Fluorescent lights are much cooler colors 
in the spectrum, and generate less heat. But if 
your light source is so close to the acetate... 
remember that acetate is plastic. You'll need to 
figure out a way to keep the light source from 
not destroying the acetate over time; I think 
that might happen long before the paint becomes 
an issue.

Another thing you might look at is placing one of 
your cels over a light source, like a desk lamp. 
You'll notice that while the paint looks smooth 
and even, looking at it normally, when it's lit 
from behind, the colors turn irregular and 
blotchy. That's with a bright light. If you can 
keep the light toned down, perhaps that might 
diminish the effect.

It sounds like an interesting project, though! 
Good luck with it, and let us know what you come 
up with. :o)

-Cres