I'd be concerned about getting the belt wet and dirty and then have it wear out fast in the boonies. If you get that thing covered in slimy mud will it slip under high torque? Does it expand or contract when really wet?
The automotive world is moving back to chains for the timing belts and when you see belts in a car they are usually covered with a plastic cover. If you had such a cover for the belt on the bike I'd feel better but I still wonder what the belt would do in high humidity. I bet it's quiet but reliability has to be your #1 concern.