If its designed to be invertible, then go for a different shape...only way I can see it being invertible is if it has six wheels a side, with four touching the ground in either orientation, or if you have absolutely massive wheels at the rear with different gear ratios on each one.

If I were to design something along the same lines (in answer to your question), Id have done it like Sunflower (http://www.happyrobots.com/sunflower3.htmlhttp://www.happyrobots.com/sunflower3.html), with the six wheel a side idea.