Can you apply something like the invisible shield on the laptop and still have the incase hard case on it? I am not sure if that'll work.
Painting it sounds like a great idea, but you at least considered the possibility of it coming off on the macbook. My question would be this. If the incase hard case were going to rub so much that the paint comes off wouldn't people who used it complain the hard case scratches their macbooks? Think about it. Rubbing would cause scratching and I haven't heard reports of that thing scratching macbooks yet. I'd almost say if you spray paint the inside, apply a clear coat sealant/protectant after the fact and make sure you give it a few days to dry and set.