Warnings first: Do with this under your own judgment only. No warranties given or implied.
That said, I just pulled my cable out, plugged it in to USB port and measure 9V with the pin positive.
That is, of course, unloaded and without any potential negotiations which makes it a little surprising to me. I thought 'dumb' USB power was supposed to be 5V until negotiated otherwise. The green light at the USB A end was illuminated and perhaps that circuitry is doing the negotiation with the brick and just delivering dumb 9V down the wire.
Good luck
PS: I think you already suggested you've done this, but just in case: You can, of course, charge the battery cells externally instead of through the RD cable. But that won't necessarily help a startup sequence (which I didn't recall being there)