I just ordered an "indicator warning buzzer" on ebay. I've always hesitated to put beepers on the turn signals, but this one has an adjustable delay before it starts beeping. While not a cancel, it will let you know the signal is still on. Seems a good compromise.
I forget what bike it is, but I saw one with signal cancel that was on a timer, but only started after you had moved so many feet, or something to that effect.
2014 V-Strom 1000 - Desert Khaki
2011 Burgman 400
2007 Honda Helix
2013 V-Strom 650 - sold