I too like using a water bladder when I backpack; I tend to drink much more that way than if I have to stop and dig out a water bottle.

I'd suggest taking the bladder out of its pack (if you weren't already going to do that) to save weight, then put it in one of the upper pockets of the Jansport - wherever it fits best. A center location will keep your load balanced better side to side, so maybe even put it in the main compartment on top of everything else, with the tube coming out between zipper stops at the top.