Pianoteq just released version 7.5 with an experimental JSON-RPC API. It is not OSC directly, but it can be mapped to anything.
Very basic embedded documentation is displayed when accessing the APIs port via a browser (plain HTTP). Pianoteq must be run with the option “--serve IP:PORT” (e.g. “--serve 127.0.0.1:8081”).
According to the basic docs, it currently provides access to the audio device activation, preset listing/switching/loading/saving, adjustment of parameters and MIDI playback control. More might be hidden though and (let’s hope) more will possibly come.