It still sounds to me like you are trying to bypass the GP paradigm. In a live situation, you should have widgets to control the parameters of plugins that you would want to change during a show. All other parameters should be preset and for significantly different sounds, one uses different rackspaces.
You can control almost any aspect of GP via OSC. That includes setting/getting widget values, switching songs/parts/rackspaces/variations, turning on recording, sending MIDI messages over OSC to actually play GP from an OSC device if desired. You can certainly control the parameters of all synth plugins directly from OSC (the synths themselves do not have to have their own OSC support)