Pretty clear, actually; I've seen sticks that are so transparent, they're almost like glass. Certainly, you'd be able to get LEDs to shine through them.
i would advise against using it. you are better of looking for a material where you can get at least a certain proof that the material is not contaminated with stuff you dont want in your body. there are many plastic types that are bio-compatible, somewhat transparent and available from sources that got FDA approval.
PLA (polylactic acid) is a clear bioplastic that I've worked with; it melts at a bit higher than 160 degrees celsius, and it is used for bioproofing medical implants. Standard-grade stuff is pretty easy to get hold of, as it is used by the 3D printing communities. In fact, I've got a reel sitting on my shelf. (Not medical grade though.)
The heat should not be a factor. I think medical grade silicon sounds right. We also have a someone that may bioproof it for us that has access to that sort of equipment.
Comments
If you take reasonable steps to keep the moulding process hygienic, you can then sterilise the final encapsulated device with povidone iodine.
J