Tue Mar 01, 2016 3:19 am
You may not like the answer... The +5V rail seems to be directly linked to the +5V USB output (at least on my A+).
The only thing I can suggest is that you get a short male to female USB lead, strip a section of the insulation off the middle (preferably without severing the wires inside), cut and strip the +5V wire (usually red), and put a NPN transistor in there, with the base pin hooked up to a GPIO pin (plus whatever resistors may be needed). You'll need to run a program on startup to trigger the transistor and start powering the USB cable (especially if you're using a wifi dongle / keyboard to communicate with the Pi), but it should give you what you want.
[EDIT]
If you don't care about the power ever getting to the hub, or anything else, ignore the transistor part, and just cut a section out of the +5V wire and tape it back up. I've done that before. You'll still need the other 3 to communicate though.