Have tried it using Hypalon dry bags. It works as long as the bag holds air, but any failure to seal will let the bag sag. As someone noted, the air pressure makes the bag cylindrical, putting the center of mass further from your back - an inefficient carrying situation. OK for portages of reasonable distance but not something I would like to haul all day.
Something else to consider. In backpacking, you normally ascend and descend constantly. On some trails, altitude can change 3,000 feet or more during the day. As you ascend, pressure in the bag increases relative to the barometric pressure outside, and the bag is more likely to bleed air. As you descend, whether the bag leaks or not, the relative air pressure in the bag will decrease, causing it to sag. The only way to keep pressure up would be to pump it or to open and re-close the bag to normalize the pressure.