Official comment
Sorry guys, there's no work-around because that parameter is not assignable within MIDI. There are a few functions we've asked the engineers to make available and hopefully we'll see them included in future updates.
Currently there are also no controllers with "extra" knobs and all R-series controllers are fully mapped to other functions (even in single-FX, as you describe, uses all 3 FX knobs for parameters).
I'll pass this along.