As for the bag thing, I think the interperitation of attaching the bag is not to be just placed on the halter and that's it. Yes.. definitely rubbing it all over and going from there! My assumption without further explination can come across a little less explanitory. If you do have a spooky horse, then it may be best to start small and work your way up.
Again, this is another "round pen" type exercise, where they don't have free roam to hurt themselves. If you keep a controlled environment in place, and take steps up to it as with anything else, then it is safe and benificial. I think that this bag trick is very useful as long as a person uses "sound" and "good" common sense in application. Just be safe and know your horses limits before going down that road!
I will say, there are crazier things done, and many I witness regularly that go beyond a bag in desensitization. Some I hope to share as a surprise in the professional and work related area, in the future.
Bells... That is true.. I had not related that to the use of bells and horses before! LOL! Good one!