I silkscreened some larger plastics and they turned out pretty decent, but I replaced them as soon as they were available from Marco. Here's what I learned:

1. Cut and refine the shape first, before printing on it
2. Use the finest screen you can get, for better detail
3. I got a Blick silkscreen kit, which came with C, M, Y and black inks and mixed colors myself.
4. Print reverse images on the underside of the plastic
5. Print black first
6. The inks are meant for paper, so give them extra time to dry between colors. Don't get fabric inks.
7. Always finish with a white shape that covers all the printed area - it helps even out the look and translucency.
8. It's an expensive, messy and time-consuming process and you may consider having Ponoko.com or some other online service print you some reverse stickers printed on clear adhesive acetate and trim them to the correct shape

I used photo emulsion. It adds about eight more steps to the process but produces better results.

I would think opaque - it will probably have better, more even coverage and even though you're printing in layers, they're very thin layers, so the light wouldn't get completely blocked. Good luck!