I remember that I have read about it mostly in the 6.12 pre-releases and thought it was released with the official v6.12 release.
I have now searched among some pre-release threads and here an example by another user from the v.6.12+dev0707 thread:
This however is not the only example. I just can't find others now but I remember that multiple users in various threads posted screenshots with something like this set up and I couldn't replicate it.
So is this not yet possible in Reaper v6.13?