The Voodoo Labs Pedal Power 2 can either provide out you with two 9 volt feeds or one 18 volt feed off AC (they make a special cable for the Q-Tron that runs off two of the normal jacks on there). I would assume you could fabricate a long cord to run the power to your bass from your rack along with the signal cord for the bass.http://www.voodoolab.com/pedalpower_2.htm
Not sure about how or why you have to mix off the + - from each battery ... or if just interrupting the ground off one of them using TRS cable would do it. An XLR cable would give you three pins to wire with, though. I assume an XLR cable can carry power since it already does for microphone phantom power use. It would be a whole lot easier to get the batteries out of the equation entirely.
I would think Rick Turner would have some thoughts on it, he's over on TGP a bit.