Make powered USB Cable for External hard-drive
Here is the USB 3.0 cable pinout.  

If you could somehow split out the 5V power/ground lines and connect them to an external power supply, I don't see why it wouldn't work.  I don't think you should connect the signal ground to the power ground, but someone else more knowledgeable than me should weigh in on that.

I don't think the host would care if zero current is being drawn, as long as current draw stays under the max allowed.  If for some reason it failed to detect the device, then you could add a dummy load resistor on the host side to fake it out.

Ensure data lines stay twisted and shielded to avoid signal degradation.