the ossia-reaper addon works as a surface-control plugin (like any other Reaper built-in surfaces, the bcf, the osc and web devices etc.). It is something that I speed-coded for a sound-installation and needs to be reworked and cleaned up a bit. It is a bit tedious to build/install if I recall correctly, mainly because you have to link it against Reaper’s WDL library.
Basically, you load the surface and most of the tracks and insert plugins parameters are exposed (I tried it with a hundred tracks and a sh*tload of vst/js plugins… so potentially thousands of parameters, it worked, except for a few crashes that I think are caused by VSTs updating their parameters tree when changing presets etc.)
Anyway, it might be easier at first to have a 8 or 16-tracks static OSC namespace with common parameters (track volume, pan, transport etc.), but I’d love to get some help with the oscquery addon, because I don’t have a lot of time right now to re-work it.
and also, as @thibaudk said, there’s now support for audio and plugins in score, so the guidelines here are to encourage users to use, test, debug this feature if possible. The Reaper addon might become a bit redundant in the future.