I went ahead and just removed the stock horn, extended the wiring, and mounted the 500Hz PIAA to the left fork pinch bolt.
I am pretty happy with it. Definitely better than the stock horn, but I think I would like the 400Hz/500Hz combo better (both for the decibels and for the dual tones).
One benefit to deleting the stock horn is slightly better airflow over the head.
'06 XT225, 2" hole in airbox, Z1 mod, 127.5 main jet, UNI air filter, Corbin seat, PIAA 500Hz, custom aluminum rear rack.