That's what I thought from looking at the ReaScript API.
JSFX can read WAV files, but can't write to them, nor pass sufficient data to ReaScript, so it seems VST would be the way to go.
It would still need a ReaScript component too, to do the muting (when indicated by the VST) and detect interaction (instructing the VST to wipe its cache). That assumes that ReaScript can read and write parameters/sliders from VSTs, which I would hope is true.