I can see possibilities of simulating a Gomboc surface on the back of a four-legged robot. However, the robot will either need small "legs" or some servo's to bring the "legs" inside the body (like a turtle) to correct its position
You'd have to recalculate the shape after that, as the interior density is no longer uniform.
Yes, you are right. Given this consideration, I think maybe the surface needs to be configurable? Not sure but it will be pretty complicated to dynamically adjust to changing density.