I don't think this is an issue in a hole. Since I only loop in shallow features (CO) I can say that your head goes directly towards the deepest part of the feature. Also, the only way to get that far around to get to that position requires the paddler to tuck very tight, thus avoiding injury. There's also somewhat of a minimum flow to even be able to pull off a loop, even in current boats. Getting pop requires a good amount of water power in a hole, and i've been in some holes that are so weak it's actually easier to flatwater loop then it is in the hole, because you sub out.
I actually split the stern open on a G-Force a few years ago. Holleywood Hole at Union is probably one of the shallowest loopable features.
Doing loop variations while going through a rapid could definetly break a neck...