This gentleman ran in to a limitation of Reapers action list.
https://www.youtube.com/watch?v=uHd9Y5oQVuk
I suggest
upping that limit to 256 channels, i.e. an midi/osc action to control the volume and pan of channels 1-256 each, instead of just 1-99 like it is now.
That said,
a more general solution could be to integrate any midi-producing device, by integrating OSCII-bot together with a frontend to midi-learn all available control resources and then mapping those on to a general OSC control surface plugin. That would be a dream for many folks. Make a bankable control surface out of any midi gear. Maybe even with auto-feedback scripting per midi-learned control. That is a much better challenge.