I actually picked up a couple of wireless baby monitors for just this type of thing. They both have two channels so if there is a real baby on the air, you can switch channels. Or you can give some mother the scare of her life when you hit a towering diminished seventh or play death metal. Should be fairly simple to bypass the microphone on the transmitter and add a guitar input in its place. Should also be easy to bypass the receiver speaker and use it as an output. Be warned - the transmitter will probably require a big battery. Each transmitter/receiver pair with two wall-wart supplies was $5 at a garage sale. Just a thought - it may not go with the cool studded leather heavy metal getup and the other stuff you wear at a gig if your guitar is hooked up to something that says "Fisher - Price".
Hmmmm..... This might be worth looking into. The adapted baby-monitor rig ($30.+) would be a fraction of the price of say an AKG or Shure system. ($100 - $600) Hopefully the results wouldn't be too lo-fi and no Spinal Tap episodes. There are also a lot of inexpensive FM transmitter/receiver kits available online.
So far: Green Ringer, LPB-1, UglyFace, and 5 ugly homemade guitars (and one ugly reso-bass)