A bit of an update.
After speaking with my GM media contact, GM says the rear battery should not be used to power any other device, including, but not limited to, two way radio systems. Ratings wise, it is about half the size of the main SLI, and like the SLI, it is an AGM.
The battery isolator uses FETs to do the requisite switching—that's a good thing—as it provides less voltage drop.
The bad news is, both batteries need to be changed at the same time. The two together, I suspect, will be over $400.
I have seen battery and it is not small so I question your source. I will look into it furthering person. Also if you pull power of main buss from rear battery the car is not going to care. GM says otherwise because average smuck is kinda clueless on hooking up things and could screw things up. I also nix that both batteries must be changed together. It is a good way to boost revenue though. The reason the suggest both is that they feel if one fails that other one may not be far behind. The 12 volt battery in volt is a regular battery and has access ports so you can use it for jumping another car.