I have heard of using 2 6V Golf Cart batteries in series before, but have not seen it in person. I did find a site that discusses that very thing. If properly connected, in series, there should be no problem with connecting them to your Pilot.
Check out... http://www.ccis.com/home/mnemeth/12volt/12volt.htm
Looks like you can increase Amp Hours significantly by going this route, but there are other alternatives, such as Deep Cycle and Gelled Deep Cycle batteries, but those are pretty $$$$.
Good luck and let me know what you end up doing.