What force does the constant speed result in? Where do I include it in my calcs?

In a traditional XC race

We weren't talking about XC racing, but distance travelled is the other variable yes. If you assume a fitter stronger rider is riding the same number of miles on their bike as a weaker one, then you have a point, but that's a) unlikely and b) you should've specified.

What force does the constant speed result in? Where do I include it in my calcs?

If riding uphill didn't require extra force in the belt, we wouldn't need to pedal harder, would we?

You have to do X amount of work to get up the hill. The chain has to go round Y times depending on how far it is and what gear you're in/have fitted, and as you will know work = force x distance moved. So the extra force is X/(Y * belt length or whatever it works out to)