Roughly 16 years ago I remember seeing a throttle hold device or maybe pitch control device that used a hall effect sensor that was monitoring head RPM, does anyone remember them or the name of them?

You talking about an RPM governor such as the Futaba GV-1 or the Aerospire Mult-Gov?

Those could be it, do you have experience with them as far as which is more programmable/versatile?

Multigov is what I use. It hooks up to a little programming screen where you set the gear ratio and the desired head speed for each flight mode and then you go fly. Others need more tinkering.

Throttle Hold btw, just so that you have your terminology correct, drives the throttle to idle and leaves the collective servo(s) to drive the blade pitch. It's activated by a switch and its main functions are to keep the heli safe while carrying it in case you bump the stick, to allow the helicopter to auto rotate for fun or in case of a tail rotor failure, and to minimize the damage if you're about to crash.