As far as I'm aware only the colours listed on the site are available.
@jonne might know differently though.
Special covers might be a good prize for a game jam.
Ordinarily they'd probably be more costly because the price of producing parts is inversely proportional to the number you want to make, i.e. the more parts you want to produce, the cheaper it is to produce them. If everyone wanted metallic covers then it might be plausible, but the main issue is guaranteeing the interest.
(Plus I think any kind of extra finish is probably going to cost slightly more anyway - materials, extra time, shipping laws (possibly).)