your could try something like the erevo trays...

maybe buy an erevo chassis, cut off the trays, cut off the lip of the dirt gaurd on the buggy and try to mound them?

Or go w/ huge batteries?

Maybe mount a roll cage kinda thing to the inside of the body (for rigidity) and mount the batteries to the cage (when body is on they rest on the base of the buggy, but when the body is removed the batteries go with it. This way, if you have a few body systems you only need to unplug take out the body clips, remove body, put next body on,. put clips in, plug in, n go.
Would require a decent amount of work to get it functional. After the design gets worked out.... maybe custom battery trays are better then this overkill idea.
Design something and contact RC Mike, I am sure he could build it, for a price.