You'd have to be able to print it with metallic ink... or you could use tshirt paint that comes in all sorts of metallics to paint it on a shirt. If you printed it on card stock, you could cut out parts of the design and sort of stencil them. Then print it again and cut out the parts you'd like a different color, and stencil those. With an exacto knife it wouldn't be super time consuming. I agree -- that would look great metallic!
I guess you could also iron it on and then just paint over it really thinnly with a silver metallic tshirt paint. It would give a metallic, shimmery look.