some guys use car battieries to run the modesl like escavators if they generally not roving around. smaller lawnmower batteries might be useful since they are a bit smaller.
an issues that will agrevate is that cars run on higher amperage than our kits - so a car battery has good amperage storage but not so good at keeping the voltage high when multiple motors attached.
therefore - you might need to run your RC electronics off of a traditional power source - motors will drop the voltage in a lawnmower battery too far and cause issues. due to this try not to use speed controls with BEC this might back feed the system .....
I like to run motor controllers - robot guys use these so they can run 24V power sytems on their 'saws of death' whil still using a low voltage RC system to control it. Check them out here
http://www.robotmarketplace.com/prod...TD-RCE220.html
The nice thing about your iidea is the added weight. many of us are adding lead and stuff to get traction whereas your idea is based around weight.
Izzy