What about using a condenser mic to pick up both the vocal and the guitar at the same time?
Isn't that an advantage of a condenser mic - that it will pick up sound a foot or two away, so that if positioned properly it could amplify both the guitar and the voice at optimal levels?

I realize this could be a problem if there's a lot of other noise on stage or in a more informal setting, but if you're the only one on stage, is that still an issue?

Genie