Thanks for the ideas guys.
This guitar is a Gibson bluesbird copy. Poplar back, 1/4" maple top. I screwed up (majorly) routing pickups, so I've patched the routes and am glueing a very thin birdseye veneer on. There are f-holes, and I've figured out how to locate them under the veneer (use a very small pin as a probe), but If I could find some way to use a very thin plastic layer as a substrate, it would give me a purfling look around the edge of the guitar side and around the f-holes. I think it would be very cool.
Here is a link to something similar from Warmoth:
Warmoth Pinstripe Accent
What I'd like to do is have a slightly thicker piece made out of shell. Hmm. just had a thought, what about copper sheeting for this layer? Or Aluminum? Might not look too bad, but I'd still prefer shell.