I'd suggest you do your own prototype and post a website. Then, you could offer them on an order-by-order (cash down in advance and to hell with the warranty) basis, and/or offer the plans and materials for sale. Remember that part of the philosphy of customisation is doing the work yourself wherever possible.
I would suggest, though, that automotive paint might not be the best effect on most macs. I have seen some very good work done with airbrush on the inside of the polypropylene shells of iBooks and PowerMacs (using the right solvents you can cleanly remove the white paint inside the clear shell).
I guess what works for PCs might not be what works for macs.
I would be interested to see what you come up with.