While there is no action, you can use this workaround.
Code:
_, tracknumber, _, fxnumber = reaper.GetFocusedFX2()
if tracknumber ~= 0 then track = reaper.GetTrack( 0, tracknumber-1 )
else track = reaper.GetMasterTrack(0)
end
reaper.TrackFX_SetEnabled(track, fxnumber, 1-(reaper.TrackFX_GetEnabled(track, fxnumber) and 1 or 0))