I find this topic very interesting.
We can currently program any new effect, but not in real time, but with a notifier that has a certain delay (a few milliseconds).
As long as the effect accepts a small delay, it can be done. However, it requires the installation of a Lua tool and that it is running (not a notifier or timer), obviously.
A particularly effective case is to dump only that new effect parameter into the master track, and thus analyze only this track by the notifier.
This is how the two new (ZMxx, ZPxx) effect parameters of Piano Roll Studio work to control the metronome (Renoise does not yet have effect parameters to control the metronome in real time, assuming that a song will always use the same beat/rhythm, when this does not have to be the case).