Okay. I'm no expert at this, but this does apply to my plans. So here's my theory:
The external USB case that I'm gonna use has a transformer that converts 110-240VAC to +5V and +12V (both at 1.5A). The power supply outputs that already so there would be no need for the transformer. The connector has 5 pins so it should just be a matter of taking a multimeter and figuring the pin out. I imagine you don't wanna be cutting up your power supply so I could recommend taking a harddive connector and making this into you power adaptor. I know +12V is running through there but not sure about the +5V. I imagine it would be. You should see four wires. Yellow, Black, Black, and Red. The blacks should be grounds and the red and Yellow (I think) it +12V so I'm assuming red will be +5V. Another thing for the mulitmeter. Make sense?
Check your USB hub. I'm going to assume that it is probably +12V. You may be able to tie this right into your vehicles power but it would probably be better to run it through the power supply.
This is my opinion and I would be willing to try it myself (I probably will once I get closer to installing). What does everyone else think?