If the Prime is attached to the dock, and the screen shuts off, so does the power to the USB.
If the Prime is not docked, the USB gives out no power.
I wifi tether my phone to my Prime 60-90 minutes every day, and I need to charge my phone while tethered because of the crazy battery drain. I'd like to charge my phone from the docks USB while I tether to keep it all self contained. I use my tablet undocked while tethering, so I can't charge my phone. I also don't want to have to use the Prime docked and make sure the screen doesn't shut off, just to charge my phone.
I haven't heard of anyone accessing the docks firmware.
Is there any way to change this?