This idea & principle should work along with DarkStar's suggestion of adapting the MIDImix.
The Midi Fighter Twister user guide at
https://s3.amazonaws.com/MF_Support_...User+Guide.pdf includes some but not all details of its Midi implementation.
Here it says "The encoder sends relative Control Change messages, a value of 65 is sent for each clockwise step, and a value of 63 is sent for each anti-clockwise step." So feedback from Reaper along with some simple increment and decrement logic based on these fader CC messages would make the encoders work.
All the device's functions seem to be based on Midi notes or CC with no sysex so OSCII-bot and some imagination should do the job.
Good luck!