Would wrapping an elastic band (a big one) around the grip a few times give a softer rubber surface to hold?
Wrap a strip of double sided Velcro around the grip and lock it onto itself.
BTW there are at least 2 different Rohloff grips - maybe one is better than the other when wet?
I like the triangular grip, in part because summer and winter I cycle with thin leather dress gloves, which can get slippery when wet.
Before I changed to the Rohloff and its original triangular grip, I used yellow knitted glassfibre workmen's gloves which came with little black spots of some rubbery, grippy stuff on the palms. I bought them at the local hunting, shooting and fishing store. Bit hot in summer but I live in Ireland where the rain didn't get the memo about taking a break in the summer...