The easiest approach would probably be to add a second optical sensor and do some math to calculate the twist rate.
https://en.wikipedia.org/wiki/SpaceOrb_360 as one example.
The stump om the base works as a 2 axis tilting joystick, and the stem of the ball works like its own 2 axis XY sliding pad with extra push/pull plus twist axes. There's a wire through the stem of the knob to get the readings. Together they capture the whole 6 axis deflections for the knob.
Trackballs OTOH is just rubbing a mousepad rolled up to be a ball to operate a Magic Mouse while charging.
https://patents.google.com/patent/US20050184962A1/en
> [..] and a plurality of Sensors located in the capacity Surrounding the track ball for detecting pressure directions.
...
> Status Expired
Therefore the cheapest way to do ball twist is to just put two sensors and use the error as the twist amount. You could use a singular high resolution highspeed camera pointed at the ball instead, but that won't be the cheapest way to do it.
Especially given the limited use kensington uses rotation for (at least to my knowledge)