I would lose the small wheels and go with a lower profile wear/scuff plate. Plastic cutting board material works well. Use bolts that have been countersunk into the plastic to secure it.
Never been a fan of piano hinges especially fer things that have a potential flex as the piano hinge flexes, it will start to bind. I've used normal hinges and taped the gap seams with Tyvek tape to keep the gaps covered up while it still allows flexing/pivoting.
When I was building my belly pan and was in the early stages of development of my rear diffuser, I had the diffuser on hinges and a few bungee cords to allow it to hang and thinking that gravity would overcome the air pressure underneath the truck. I was wrong and the diffuser ended up rising/falling based on the air pressure building up which would pushed the diffuser up until the pressure would bleed off causing the diffuser to drop. This banging cycle started around 30mph. I finally set the diffuser in a fixed position and angle but I left the hinges in place as I have a rear wheel drive and needed the flex in the rear. I believe you will probably discover this if you go with a gravity based solution.
You could go with an inner tube/air bladder between the rear diffuser and the bottom of the car body so as you contact the few driveways, it would compress enough to allow clearance and then extend back out to the proper angle. You'll have to experiment with the amount of air pressure that will meet yer needs. You will still have to install some wear skid strips so it doesn't beat up the diffuser too badly.