If this is a Rear Engine, my thoughts go to your engineering challenge in a way where I'd probably fabricate 2, not just 1, but 2 trailer hitches in the front, to a thick 1/2 inch+ plate connecting the two frame rails together, and having a custom designed motocycle carrier to plug into those two hitches, also with 1/2 plate walls. It'll be very heavy, but it could work.
The plate would go down low to the ground, and build a cage for the large bike. Two hitches for stability as well when swaying etc. Doesn't have to be too wide, just enough to get the wheel on, and then create a latch to hold it up top on the bike to the bus.