I remember from the old runequest rules that the concept of ENC was how many "Things" you could carry, with the idea that heavy things count as more than one "Thing".
Using this logic, a person who is carrying another person is so fully encumbered that there is not much else that he can carry and still move around. Thus, a person's ENC value would be approximately equal to his Size.
I think the problem is that backpacks should not be rated according to ENC, but according to size and weight. Can you really put 300 pounds of gold into a backpack and expect it not to fall apart if you pick it up? (Assuming you're strong enough to life 300 pounds, or get a crane.) Certainly it depends on how well the backpack is made.
Perhaps a good rule for backpacks is that a backpack counts as 1 ENC or an ENC equal to the SIZE equivalent of the weight contained in the backpack, whichever is higher. That way you can load a backpack up with wooden objects with no difficulty, but if you fill the thing with gold (or a chopped up adventurer), you'll be staggering around.
Note: chopped up adventurers weigh approximately 50 pounds per cubic foot, and Size must be logarithmic rather than linear, so you probably could not put an average size adventurer inside an average size backpack.
