I was going to wait until I have some demo video and test flights, but I have designed a PCB and located the parts to make a plug-in yaw mod. No soldering or hunting for parts. Just drill holes for the pot and the optional bypass switch. (Optional subject to testing)
Here's a couple of photos of the prototype. You would simply unplug the left joystick from the R/C PCB and plug this board in, then the joystick plugs into this board. You then plug in the bypass switch and the yaw rate pot. The trim pots are there to eliminate the left-right imbalance introduced by the low tolerance of the dual-ganged pot. The other jumpers are there for prototyping an ascend/descend switch.
The prototype as shown has a parts cost of $35, but in quantity it would go down. At this price, who is interested?