I have no experience in this area, but I'd say if you can hit focus in the middle of the curved plane, whether concave or convex, you'd be doing well!
I think most have the paper along the same plane as the tin - curving with it. IMAX movie screens have that curvature I believe. But I think it would be great if you experimented with having the paper bulging towards the pinhole. Could be unique?

A quick search gave me these:
http://www.paintcancamera.com/use.html
http://www.kodak.com/eknec/PageQueri...equestid=45889

Please share your results!