An option to trying to seam seal the pack would be to use a dedicated waterproof pack liner. I can only imagine how tuff it would be to do a good job of sealing a pack. And with the stretch of the fabric, I would think it would be tough to achieve 100% waterproofing when it is loaded up. The tarps, tents, gloves and stuff sacks I have seam sealed have all be less than perfect jobs, even with my best efforts.
