Currently you can control track FX through the TrackFX*() functions, but there is no native way to manupulate the FX params on the TCP/MCP, unless you use KBD_OnMainActionEx()
See
this thread for details
Some ideas:
// get # fx-params on a given track
REAPER_PLUGIN_DECLARE_APIFUNCS int (*TrackFXParam_GetCount)(MediaTrack *tr);
// get fx-param name
REAPER_PLUGIN_DECLARE_APIFUNCS bool (*TrackFXParam_GetName)(MediaTrack *tr, int fxparam, char *buf, int buflen);
// get fx-param value
REAPER_PLUGIN_DECLARE_APIFUNCS double (*TrackFXParam_GetValue)(MediaTrack *tr, int fxparam, double *minval, double *maxval);
// set fx-param value
REAPER_PLUGIN_DECLARE_APIFUNCS bool (*TrackFXParam_SetValue)(MediaTrack *tr, int fxparam, double val);
Ideally there'd also be some change notification events from Reaper.
Also i'd like to see a way to get the actual plugin from which the param was used, so you can e.g. open up the plugin GUI when manipulating an fx param.
Yves