The Camaro convertible was able to actually increase chassis rigidity without changing any struts, bushings or changing the spring rate from the Camaro coupe. GM added a series of braces underneath the car along with the strut tower brace. The additional body support is:
Strut tower brace (under the hood)
Transmission support reinforcement brace (Green)
Underbody tunnel "X" brace (Yellow, not sure if it made production)
Front and rear underbody “V” braces (Red)
The entire underbody of the convertible has been tied together.
After watching the Motor Trend video, it was said that the ZL1 would use chassis bracing and because these pieces are already being produced/used in the Camaro lineup, I would think that these will be the exact same braces (maybe beefier even).
Anyway, here is the diagram (at the bottom) and some pictures from Vert owners. I do not see the "X" brace in the live photo's, maybe the large transmission brace was used instead of the "X" brace to clean up the belly.
Transmission support reinforcement brace
Rear "V" brace
Front "V" brace
A link to the Vert's braces:
http://www.camaro5.com/forums/showthread.php?t=134218
There was additional support added to the A-pillars, an inner reinforcement bracket in the windshield header, a reinforced front hinge pillar and reinforcements inside the rockers. This was done to improve noise and vibration and contribute to the elimination of cowl/steering wheel shake. I don't know those improvements would be useful or not.