I made the PVC long enough to come right up to the edge, but if its shorter it will shift laterally. Typically I actually do these as bent knee rack pulls from different levels so I'm not moving laterally.
I thought about the towel changing the line of force, but from a physics perspective, in a normal zercher the line if force theoretically runs from the bar to the shoulder. In this setup the line of force will be somewhere between the a) center of the bar and the shoulder and b) the line drawn from teh hands to the neck. The hands to neck line is virtually the same as the bar center to shoulder line, and if anything is a little more horizontal so it just increases the force that is rotating you forward.
Anyway it kills the abs with just a few heavy reps so maybe its different but it seems to be effective.