Actually the weight of the boat is not very important because it is floating already and is actually a counter weight moving around the center of the axis of rotation, most of your mass in your lower trunk is centered around the axis of rotation and requires little force to rotate, the force or gravity you have to fight is acting hardest on the upper torso and head, because you are lifting it the greatest height and distance about the axis of rotation. That's why it is easer to do a lay-back roll, your axis of rotation for your weight from upper body is dramatically shortened. I agee however that lifting the head engages the wrong knee and kills the roll.
Right. Laying back, that is, flattening your body down toward or on the rear deck, significantly decreases rotational inertia (sometimes called moment of inertia). That's because you signficantly reduce the distance from the center of rotation (the long axis of the boat) out to the large mass of your body (and head too, of course).
And that helps explain why just keeping the head down has ~no~ appreciable effect on rolling. In moving the head back and forth, you are barely reducing its distance from the boat, maybe by an inch or so. So your are not much reducing its rotational inertia. You are mostly moving the head along the arc it describes as the body rolls up, and that's no help at all purely from the standpoint of rotational physics.
Now, if you could manage to put your head somewhere around your navel or crotch, ~that~ would help. But there are certain anatomical impediments there. ;-)))