I don't know any boaters who would willingly put a hydration pack over a PFD for paddling, for precisely the reason you mentioned-- the interference with the seatback (or sprayskirt, depending on the type of kayaking.) Most folks I know just throw a Nalgene into their boat.
The other thing to take into consideration if you're dealing with the possibility of an unintentional swim is safety-- the more webbing, straps, and loops you have attached to your body, the more opportunities there are for them to snag on obstacles.
If you were really bound and determined to have emergency water, you could always fill one of these guys (http://www.backcountry.com/platypus-platy-plus-bottle) and stuff it into a pocket in your PFD. Not as much water, obviously, but better than nothing.
Hope this information helps... feel free to hit me up if you have any other questions at all! 800.409.4502 ext 4456, or email@example.com.