If it's a 9V battery, it's dead easy to get a clip-in battery compartment you can add onto your guitar, provided you're confident about sticking a hole in the side somewhere. In that case, the 0.7V voltage drop due to the diode will not be an issue.

Graham.

PS. Apologies about thinking I'd lost my ring - and apologies to John too! Did the message reach you b4 he turned the room over looking for it?