How about something like this:
Bear in mind that this is something I whipped up in the past 10 minutes, so it's just to illustrate the principle.
A - Two "directional rings".
Possibly something is already available in rock-climbing shops. If not, something as simple as a ring of PVC tubing with nails hammered in at an angle might do the trick.
B - The frame of the whole setup
C - Metal frame carrying the ring around the dildo in front, and connected to the aft A ring.
D - Dildo
E - Rope. Needs to be somewhat rigid.
The idea being that as you deepthroat the dildo it will push the aft A-ring back, pulling the rope forward maybe ½ inch.
The other A-ring holds the rope in place, whilst the first ring slides back over the rope as you pull your head back, thus gripping the rope ½ inch further down.
One problem that springs to mind is ensuring that one can't pull the rope very far each time.
Otherwise the rope will simply slacken between the two A-rings.
Still just a rough idea, but the principle seems sound.
Or maybe I'm just incredibly dense.......Which in all honesty wouldn't be the first time.
CheerfullyInsane
PS:
I was kinda hoping for thatWe're all a little loopy here.