Hmmm, yeah I was thinking about that. Is usb communication unaffected by the absence of the V+ connection?
Don't know for sure, I've never tried it. The USB ground, and two data lines are all that's need for the USB to function, assuming the USB device has it's own +5vdc voltage source and it's power source ground it wired to the USB ground wire. But possibly the PC side needs to see a minimum amount of current flow to decide that something is trying to attach when you plug the cable into the PC?
You may have to wire say a 50 or 100 ohm resistor from the USB +5vdc wire to the USB ground wire on the PC side of the USB cable where you cut the +5vdc USB wire open. That should work for sure.