I'll admit to almost total ignorance on this subject, but all of the guys I know have a receptacle/box thru the sidewall of the guitar where you can stick in a new battery.

Most of them appear to be wired so that unplugging from the amp disconnects the battery. (At least most of their cursing comes from running the battery down when they forget to unplug between sessions.)

My S.O. has an old Washburn bass with this setup, and I'm pretty sure a couple of the ones I've seen are Martin factory setups. Pickup style/mfr unknown on these.

Shouldn't be too hard to install, even if you have to salvage one somewhere. Ones I've seen were all 9V, but if you could change the battery easily you might be able to "throw away" enough battery life to put a regulated dropping circuit inside if you need some other voltage?
John