I sell and install the Viair compressors that go with on-board air or train horns. I wouldn't install a compressor under the seat. Others may have done it, and it may be okay, but that point is pretty low and if the compressor goes under water or gets too much moisture in it, you'll have problems. I've seen a few compressors mounted to the side of the rear racks close to the rear fenders. There's a little room there, but a tank would have to go elsewhere.
Funny, I'm doing a compressor now, Just for pumping up tires. I'm mounting it on the drivers side up above the fuse box. It's one of those cheap no tank units so if it fails, no big deal. Leaving enough wire to walk it around the vehicle since the hose is rather short. I'll post pics later