If one wanted to add just USB charging ports you could use this hub by Carnetix found at http://store.mp3car.com/DC_Powered_4..._p/adt-015.htm
. This can be configured to power on with the ignition if you like or it will turn on if connected to a down stream USB connection like your laptop. If you wanted to hide this and just make the ports seen you could add these
Now you run into the real problem with power on USB, A lot of devices that use USB cables for charging may want more than the standard 500mA per port USB spec. You can use a cable such as this http://www.amazon.com/Apricorn-Power.../ref=pd_cp_e_3
this will double your power.
There are also some other 12 volt USB hubs on the market, the Carnetix looks to have the most features if you chose to make it a stand alone install. ( I do not own the Carnetix hub)
So this could be an option.