A few years ago, I played around with printing platinum on vellum. It worked reasonably well. If I remember correctly, it required less emulsion than water color paper. The effect was interesting when put on a light box.

I remember reading about some platinum printer who would buy old parchment--normally gregorian chants and print photographs of cathedrals directly on the old parchment. Sorry, I don't remember the reference.

Jed Devine did a lot of experimenting on different vellums and other translucent papers. You might google him and see what you can find.