I have idea for balance, and eliminating combat exploits (which i think is the main issue, cause implementing backpacks as a normal item, will bring more 1st alts):
you can use backpack in slot of armor. And the, item adds you carryweight. This way, you simply cannot fight (or its really painful) while hauling.
The additional advantage of this solution is: opponent can see you wearing backpack( awareness), and that gives info about your purpose on particular location.