Interesting, many people propose putting insulation on top of their roofs (instead of against the ceiling inside) in order to save headroom. Nobody has ever actually done it as far as I know, though. On a school bus with a metal body this would not function very well as insulation, thanks to the high thermal conductivity of steel, but on a shuttle bus like yours (which I think is literally just cardboard and fiberglass for the roof part) it would provide effective insulation.
If you could keep the stuff attached, anyway. I think that's the real problem with this, is that you'd basically have to build a second roof on top of your insulation layer. Something like gluing XPS foam board to the roof and painting it with Henry's Tropicool might last a few a hundred miles before pieces start coming off.