I have no firsthand experience or knowledge of belts, but my knowledge of bike mechanics in general suggests that the probability of failure of a timing belt (or chain) is low. Very low. The chain drive to the rear wheel probably has a higher probability of failure and I'm guessing a chain breaker is not part of your field repair kit. If the belt is as susceptible to damage as is being alluded to, I for one wouldn't think it worth carrying, but if I did I would pack it last, on top of everything else in the pannier. Folded (correctly) in an appropriate size of Ziploc bag.