@ James - Pressing the Reset button on the G400 within 1 second after deployment still fails.
I found a major clue (and workaround). I noticed that when I plug the G400 into a USB 3.0 port on my laptop, an entry for a ballpoint serial mouse sometimes appears inside Device Manager, in addition to the GHI NETMF Debug interface (screenshot). Deployment fails in this situation.
If I connect the G400 to a USB 2.0 port, deployment consistently works (fingers crossed).
And the really nutty thing: if I disconnect AC power from my laptop (which causes the CPU to throttle back and the PC to implement several power-saving features), I can deploy successfully via USB 3.0.
Something tells me some really nasty OS or USB handshake bug is lurking around somewhere.