You might consult with some fine artists. I know there were some at my college who were somehow printing on all kinds of weird materials. I have no idea how it was being done, but based on the gallery, they didn't seem to have much in the way of limits concerning media.

After a bit of searching, I ran across this company: Print to Anything Solutions
They claim that their flatbed printer can print on anything. If I understand the machine correctly, the printhead moves in 2 dimensions over stationary a medium instead of using a drive to move the medium itself, which should lend itself to printing on any flat material that fits in the bed. Whether or not the material can take the ink is another matter, of course.