Seth,
I don't see why you would want this. Surely if you're trenching, you want the clamp to hold the spacer in place, and then hold the workpiece by hand. When trenching, the piece needs moving after every cut, and there could be dozens of passes needed.
I can't see anyone wanting to clamp the workpiece, make a cut, unclamp it, move it 2mm, clamp it again, make a cut, unclamp it, move it 2mm, reclamp it, make a cut, unclamp it, move it 2mm, reclamp it.....etc etc.
Unless of course you're trenching very small pieces? In which case, I'd do the trenching part on a longer piece and then cut it shorter.