So are you planning to run motorcycle tires on the front with the 21's, and can you do that without assuming extra liability that might make insurance "interesting"? Again, just wondering about the approach.

The wheels and tires on a Hiboy are one of the defining aspects of the car. Normally one picks the wheels & tires as a first step, then designs the suspension to work with the ride height that's desired/required. Seems like perhaps you're looking at suspension first, and that's driving your wheel/tire selection?