The Maxbrake could also be mounted at the rear of the bus. The only thing you couldn't do is manually actuate the trailer brakes from the driver's seat. With a bus you're probably not going to have issues with the trailer getting out of control.
I did look into air brakes when I replaced the brakes on my trailer. Dexter only supplies air brakes on 10K and up axles. I don't know that air brakes would really work with a bus anyhow. Our brake systems aren't designed to supply air to an additional two axles. I would have severely limited myself on vehicles that could tow the trailer if air brakes were even an option.