My helmet generally goes on my right mirror on my Vstrom and is pretty secure there, but the one issue I do run into is if after I pump gas I want to push my bike over to a parking spot, the way the helmet sits on the mirror keeps me from turning my bars left. That means I usually have to put it on the seat which leaves it very vulnerable. I've started to carry one of those nets stretched out over my back seat just so I can slide the helmet under there and it will secure. Of course if I have anything back there, that doesn't work.
The other big issue with the mirror is on a long trip, it can become quite nasty with bugs and then all that stuff ends up inside your helmet.
So yeah, I would definitely be interested in a $10 device that gave me a quick and easy place to put my helmet while I'm filling up and keep it from hitting the ground.