never seen, but it's obviously possible. i think that would be amazing assuming you had a nice design to put on it. if it was a bad design well...what a waste 😛.
if i ever get a G5 Powerbook, I will definitely look into personalizing it in this way (custom paint job too!).
that being said, there is a web site that does custom painting of powerbooks, forgot the name, but they looked excellent.