I think it sounds like a fun idea. I'd be a little hesitant about using old Nady units, though. I don't know if you're aware of it or not, but in June of this year in conjunction with the DTV changeover
the FCC changed the rules about which frequencies you can use for such things, so short-haul units in the 698-806MHz band are illegal now. I seriously doubt they're lurking around waiting for someone to light one up so they can pounce on them, but you might find yourself the unwitting receiver of some broadcasts that end up flying out of your amp's mouth at high volume at inopportune moments. And if you interfere with somebody else's radio setup, they might pitch a bitch, in which case the FCC
will come after you. I think it's public service/safety communications that those frequencies have been sold/assigned to, but you'd have to look it up to find out for sure. Doesn't really matter - what matters is you can't use them. The Nadys, Sennheisers, Shures
et al of this world didn't put up too much of a fuss because it meant they were going to get to sell a boatload of new equipment that operates in a different frequency bandwidth. Just a heads-up.
The other thing I'd caution against is putting a switch on the back of the guitar. Sure as sunrise, no matter how you orient it, you're going to wipe it on or off at the wrong time. Put the switch on the front, and use a miniature multi-pole that turns power off/on and switches the standard output jack in/out of the circuit at the same time.
