Remember these are BOF vehicles not unibody, so this has no effect on the frame at all.
True, keep in mind though that the frame and the body are separate, so when you remove a key structural component of the body it will weaken. Did the builder take to proper steps to reinforce the door jambs, what about the space in between which now has no structure to keep it together. Unibody or BOF, the body of the vehicle still plays in the structural rigidity of the vehicle. MAin reason for extra bracing with a sunroof, not only for weight but you have a 2-4 sq ft area missing from the roof this invites twisting, warping and other things that do not bode well for metal.
I think I seen the PT. Now I wonder if it was to be made water proof would it be like one of those Hummer stretch limos with the hot tub in the back.
Two things I notice. One the lift gate is welded so that would help some with rigidity, but not enough to compensate for the entire roof. Two, I bet people in the back seat, that is if this thing does do high way speed, really take a beating from wind.