Hey Rob -
Our 2015 Seneca has an air horn that's tapped into the chassis air system, so it is possible. The original owner (who took one trip and brought it back, leading to a great deal for us) had it added at a Freightliner shop in Florida.
You're absolutely right - the stock horn sounds like it belongs on a smart car. The air horn sounds like a train and definitely gets the attention of the offending driver.