There would be a lot of practical problems with the idea.
Its technically possible, but not easy.
I'll bite on an actual answer why not.
I'm not sure how the automotive AC pumps work - do they run constantly and have internal regulation, or are they on-demand with a clutch, and they only come on when the AC demands? (You might need the pump to communicate with rest of the AC system.)
Either way, you'd have to find a way to un-belt the pump from the bus engine, and hook it up to the external engine - otherwise you'll be trying to spin over parts of the engine with the AC pulley/belt. I wouldn't be surprised if there are multiple things on that belt other then the AC pump, and the belt probably isn't designed to be removed and replaced all that frequently.
On top of that, you'd need to make room for the aux engine under the hood, and make sure that it was oriented so that the pulley spins in the right direction, and is on the correct side of the engine. (The engine controls might not be on the side of the engine facing outwards....) You would also have to make sure that the RPMs match (or install the correct pulley to compensate.)
Personally, I'd just buy a generator. Coiling up a 50' extension cord sounds a lot easier in the long run. Plus, with the generator, you can run other things beside the AC.