David,
I think someone recently tried the metal sheets from Booksmart Studios or was about to on the Z3100. Maybe they'll report experiences. The paper path from the back of the printer, not the normal sheet feed, is fairly direct.
On their website Booksmart Studios says they're working on profiles for HP. Maybe they're for the Z. It wouldn't hurt to contact them. They're generally extremely helpful. They post in these forums from time to time.