I've done something similar with my set up. Instead of routing additional usb ports from the 12v to the front, I routed a 12v splitter to the front so that I can have 12v access in the front compartment. That way I can use additional 12v accessories whether it's for more usb ports or for other future needs.
I'm using the splitter below from amazon that has 2x12v and 2 usb built in.
http://a.co/d/2jYBsSw