I would start by weighing the bus, including individual axle weights, and see how much room you have left on the front axle. If you do the math and can put the 700 pound bike on the front without overloading the axle, I see no issues with doing it.
The front bumper on my bus is very stout. I put a bolt-on hitch receiver on mine, and one of those hitch based cargo carriers to carry my generator and 3 5 gallon fuel tanks without any issues.
Similar bolt on hitch receiver from amazon
I'll just add that it took 2 of us to remove and re-install that bumper, and it took most of an afternoon to do the entire job. Drilling the holes was a very tiring process. That steel is very strong.
I would think a hitch mounted motorcycle carrier would be able to work with this reasonably well.
You could also use 2 or 3 of these, and some basic receiver tubes (available online) and build your own platform atop those. As long as you don't exceed the GAWR for the front axle, and don't impair the driver's ability to see out the window, you should be golden.
Hope this helps,