I've been digging through a lot of docs and videos already, but thanks anyway. One can never have enought of that.
Anyway, I fiddled around with soft takeover, but that doesn't work across tabs so that's out the window.
I found this video:
https://www.youtube.com/watch?v=p0LBdXXcg7g
But is it really that convoluted to get endless encoders working like this in Reaper? The video is more than 1½ year old so I'm hoping that won't be the case anymore...