In reply to NONACK:
But triangulation would be through the trunk. Perhaps a roll-bar with rear stays that pass through the rear deck into the trunk and then bolt through the floor to the frame?
I don't know... what I see there doesn't give me warm fuzzies... 
Part of the problem is trying to reinforce a part of the chassis that was designed to crush upon impact and do little more than hold up the bumper and trunk.
Another thought would be vertical sheets of metal in the trunk that would be bolted through the trunk pan into the frame. Sort of like making the chassis the bottom section of an I-beam, if that makes any sense.
