So between the ProCharger and the Air Ride system the bike has additional hardware added everywhere that a mandated safety item needs to go without any consideration for where that mandated safety item should move to. Since it's the ProCharger that actually occupied the horn space I'm surprised some alternative mounting isn't supplied by them.
The only places I've ever seen horns mounted (electric or air) that you've not already filled apparently are:
1) inside the fairing
2) beneath the tour pak
3 between the front frame rails
Maybe you could also lose the rear fender filler strips and mount on the rear frame or saddlebag mount brackets in the rear?
There's just not that much real estate to work with. You know the spaces available as well as anyone else here. Maybe just keep a canned air horn in your pocket and pull that sucker out whenever you need it

?