I once read a Louie Simmons article where he mention pulling the sled with the neck harness. His advice was to stand facing the sled and do a good morning motion, pulling with the head strap.
If you have poor posture, pulling the sled forward could reinforce, or "strengthen" the forward slump.
Pulling the sled sideways with the headtstraps seems like brutal torture. I bet it would be really effective.
In wrestling practice back in high school we to have our partners "push" our heads while we resisted with our necks. We also did lots of bridging on our heads. Lots of "isometric" or "static" work. I think the sled would work too.