One way, albeit silly, for the car to do regenerative(or at least not-friction) braking with the battery at 100% would be to intentionally(and automatically) enable the electrical accessories(resistive heater >and< AC for the cabin, seat heaters, defrosters, and so on). Diesel electric locomotives have entire panels of electric resistance heaters to dissipate the energy generated as they slow down.
I imagine the scale of the slowdown for a 3 with all its electric accessories enabled would still be pretty dismal, IIRC, the heaters are like 6kw. I don't know what the regeneration-side of the power graph is, but I'd be surprised if 6kw was more than 1/3 of the usual regeneration.
Way simpler to just charge to 90-95%...