Well I don't know. Everyone who reads the post now knows it including you, but I bet more people know it than you think and even if they don't I'm still betting that a rhythmic, continuous signal will draw more attention than just a blasting whistle. If I'm up a tree and here some fool blowing a whistle I'm probably just going to be mad but if I here a pattern that repeats over and over again I think I'd be more likely to wonder what the heck is going on and maybe get down and go investigate.

Anybody who's ever been in the military should probably recognize SOS and I'm pretty certain if GW or any other law enforcement is within hearing he'll come running.