I use the bus racks all the time with my DF. I'm trying to picture your problem- is the swing arm pushing your Giro backwards because of the tension? Instead of trying to shim the front wheel, would it be feasible to immobilize the rear? Use one of those ring/wheel locks like the AXA Defender maybe?

I do have a couple of quick questions that you can give factual answers to:

1)When you put your Giro on the rack, is there still room for someone to use the other slot?
2)Do any drivers complain about the seat(back) impeding their line of sight?