I recently came across this:
http://www.sodis.ch/methode/index_ENYou can get clear water with using a shirt as a filter. And then carry a pet bottle on the outside of your backpack or anywere else. Filtration while on the go.
I dunno if it works with the water sloshing about during the ride and you need to have a solid system of changing bottles and keeping tabs on clean and dirty water.