Yeah, and your point is quite correct - on paper. I was simply envisioning what we typically see with the majority of blocks in the back - pushing. However, if the blocker was to, indeed, throw a shoulder into the middle of opponent's back, then yes, technically, that woukld be an illegal block in the back.
It is noteworthy that a similar administration occurs in the exceptions to a clipping foul.
Under 9-1-2-d:
3. When a player attempts to reach a runner or legally attempts to
recover or catch a fumble, a muff, a backward pass, a kick or a
touched forward pass, he may
push an opponent below the waist at
or to the buttocks (Rule 9-3-3-c, Exception 3) (A.R. 6-3-1-III).
4. When an eligible player behind the neutral zone
pushes an opponent
below the waist at or to the buttocks to get to a forward pass (Rule
9-3-3-c, Exception 5).
I interpret those to mean that a player cannot block with a shoulder anywhere below the waist. However, he can push with his hands at or above the butt, but not below the butt.