Well, there is a solid mounting bolt that holds the stock horn, under the headlight. I was just there installing the fog light kit (bought my bike before Triumph stopped nickel-and-diming riders and actually included those in the package). Bending some sort of bracket and mounting the horn there should be doable.
Just don't use the stock horn cables, won't feed enough power, needs a relay. Otherwise, maybe you could fit the compressor under the seat? Hmm, maybe not enough space.
There might be space in the wiring area under the tank assuming the compressor can lay flat and you get something bike-sized, not truck-sized. If you get a chrome two-horn solution and create a good bracket where the stock horn is you could have them pointing straight out front even, for that maximum in your face thing.
Just brainstorming.