My credit card was out on the desk when I got this response to my query of the Promote Control people:
"Unfortunately, no. 5D2, 7D and 1Dx cameras have a loophole in their USB logic allowing us to exploit it and force camera transition into Bulb mode via USB. Both 5D3 and 6D cameras don't have that and thus this transition via USB is impossible at this point. Thank you."
Oh, well. That really was the only feature that I wanted, and was willing to buy the unit.