Maintenance modems and power failures

Sean Donelan <SEAN@SDG.DRA.COM> asks:

Most network providers are aware of the secret hiding in most of our
POPs. Those little sporters and courier modems stuffed into a rack
somewhere plugged into the nearest available outlet, as the only
out-of-band access. Has anyone come up with a solution for powering
these forgotten pieces of equipment, which doesn't cost more than
the modem itself?

in our machine room, the maintenance modem is a dual powered,
9v battery and AC wall wart, "2400 bps Mini-Modem".

I got in years ago, and have no idea if they are still available.
Other than "2400 bps Mini Modem" and "Made in Tiawan" neither
the modem nor manual have any markings suggesting a manufacturer.