Hey,
I don't know if anyone has tried this before. But, rather than using a mirror, or trying to get your whole head inside a 5" porthole, why not use a digital camera.

I just did it to check out a new captive bolt assembly that I put into a nacra for someone and it worked great. Just put a piece of masking tape over most of the flash or it will be too bright.

Pretty cool, you can get a perfect shot of some inside detail with perfect lighting.