I don't think putting it on the helmet strap is a good idea. If you're incapacitated a helmet is likely to be cut off so they can get you onto a backboard, and left at the scene.

There's an ongoing thread about this on one of the motorcycle boards I'm on, and the consensus from several EMTs is that they do not look for this stuff. At all. They will stabilize you and get you to a hospital is all. I think you do want it when you get to the hospital, though. They will need a next of kin (or medical POA) to make decisions, drug allergy info, and an insurance #.