The short answer is yes. The long answer is that you will want to improvise a way to keep the bladder upright and segregated from your gear. I have mountain biked with many people who have just thrown a bladder in their bookbag for long rides. I would also check the classifieds at mtbr.com. You might find a good system for a song.
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Two is one, one is none. That is why I carry three.