To clarify some more, the (wired) lavalier just won't work well, no matter how it's rigged. The subject is overweight and has a hollow body guitar against his chest while sitting down, therefore, there is no space between the mic, the shirt and the instrument, and it rubs, even if it's taped and everything else, everything is still rubbing against everything else. So the lavalier is out, I really don't want to see another one of those things again. I will look up some of the other solutions you've presented now..

You actually should probably use a couple or three mics, one for the vocal and another one or two close to the guitar to pick it up properly. I say one or two because there are different techniques one might use to mic a guitar depending on the nature of the instrument. If it's acoustic, one technique that's sometimes used is to clip a lav onto the edge of the guitar's sound hole, so even if you get another mic such as a hyper to use for the talent's voice, your lav won't necessarily sit idle. Of course, there's no need to record both his discussions and his demonstrations in the same continuous take, in fact there's a lot to be said for shooting it using establishing shot, closeup etc sequences assembled from multiple setups the same way a dramatic production would be put together.

I just got off the phone with Rode about the VideoMic and it appears to be exactly what I'm looking for. As far as the last suggestion, the only thing I need the mic for is the voice. The instrument is run directly into the camera, no mics needed. From what the guy was saying on the phone, the Rode VideoMic seems to be the way to go =)