I still think the solution IS a throttle lock of some sort. I know you say it only happens at 75 indicated, but that could be a due to pressure needed to hold throttle open that much?
I use the NEP MCC20-P version throttle lock. It is only 20$ or a bit more and solves the numb hands thing for me 100%.
In my case, it has nothing to do with vibration or speed, but the twisting needed to hold throttle open. I had the problem on Gold Wings, if I did not use cruise control, same thing on Tenere if cruise not used, and now on AT.
The NEP goes right on in 5 minutes if you get the right version, easy as pie to set and release, and I am betting it is a great solution for numb hands. It is for me!
When I drive out on open highway, I use cruise control on our cages also. Why use muscle tension to resist spring mechanical tension when it is not necessary????