I'm curious too, but I've tried internal-mic's; external-mic's clipped to the soundhole, a combo of both, sound-hole pop-ins of many brands, several under-saddle piezos, personally hand-made under-top piezos, stick-ons, stick-ins, magnetics, crystals, transducers, ribbons (once I put four different mic-types inside and outside my National-steel with a blender)......different degrees of success (and failure

). Double boom-stand mics (sound-hole and bridge) is THE standard to aim for. If you have something that'll sound like that, the world will beat a path to your door. (The world of acoustic purity anyway!). Good luck!
(A good electric harmonica wouldn't hurt either)
(Yeah, tried that too.....

)
The "poor man's patent" Wave mentioned above is good
supporting evidence, but not always sufficient in and of itself. It needs to be dated with no possibility of date-tampering (like sending yourself an empty envelope and then later putting your idea in it and sealing it, etc.) Patents are expensive to get. Another way to date it
with authority is this: Like I said, patents are hard and expensive to get. However, copywriting is not. Of course copywriting is for recorded or written material, so what do you do? You describe your invention/idea/whatever in great detail and very graphically on tape as you record yourself. You get a form from the Library of Congress:
CODE
The Library of Congress
Copyright Office
101 Independence Avenue, S.E.
Washington, D. C. 20559-6000
L of C Link.....then you submit your new original recording (like titled as "Ode To My New Invention...The Super-Duper Ultimate Mic" or whatever)....(this used to cost $25, might be a little more now. It USED to be free!). Anyway, NOW you have it dated
with authority. This HAS held up in court several times (although grudgingly by the presiding judge). Added to your "poor man's patent" and any other documentation (like a video of you describing and showing it while the TV is on in the background behind you with the evening news for December 11, 2003, with your two year-old walking around, etc). Again, that video is purely
supplemental evidence (could be a pre-recorded video of the news possibly, hard to pre-record a two year-old though

), but it
supports your other evidence. When taken together with the government-documented copyright date, you have a preponderence of evidence to support your claim. Anyway, good luck! (Hope it works. I'll buy one!)