Hmmm, I wonder whether you could use a process like carbon printing and use the appropriate metals instead of carbon in the gelatin mixture. The final transfer would be onto the unglazed ceramic. I suppose some sort of clear glaze would need to go on top (?). Essentially, you'd be making the decal with gelatin, dichromate and the appropriate metal powder. Then on the other hand, it might just all run off too.

Warning: this comment is from an engineer with almost no experience in ceramics and only a bit more with carbon printing!!