Oh man, thats doable too. Imagine how loud that would be?
I have way to much on my plate but have been thinking about some sort of compressor powered by friction from a turning wheel. A small chamber with a pressure release valve so as not to over fill. Attached to a horn. The compressor would be about the size of the old bike generators. Nothing big. Secure it to the front forks. Dunno, just rambling.