I have a Screaming Banshee on order (the new Plug & Play version) which seems to be the ticket for this. It is a Stebel-like air horn, but it is designed with a controller relay that fits right in with the stock horn, so you don't need to bypass it. If you tap the horn button, you get the stock horn activated. However, if you hold the button (still under a fraction of a second) it will activate the air horn too. You also have the option of integrating the system with your headlight so it flashes the high beam for a little extra attention. I have not received it yet and therefore do not have direct experience with this system, but check out the information and the videos on their web site. I have searched for some time for a good horn system to use on my bikes and small car, and this seems to be the best thing out there in terms of functionality:
Just a clarification, the new Plug & Play Screamer system from this company is not an air horn, but still produces a blast of 120+ dB measured at 1 meter.