would be nice to grab the external synth too with instrument grabber 
for the same reasons for which we cannot render audio coming from external synths when rendering XRNS to WAV, this cannot be done.
Not entirely sure i see why ?
They are completely different things
Bungle
Technically they do the same thing.
Currently renoise disable the audiocard when rendering.
This must change before we can think about grabbing HW instruments.