I don't think the 520gU was really intended to supply much power over USB, a bus-powered USB-HD is probably pushing it. Remember that the router was only designed to support a USB printer, which likely requires almost no bus power at all!
I don't think I would put more than 5V on the USB port. While USB devices might have an internal regulator, you don't know how much voltage they are made to accept on the input. Things like capacitor voltage ratings start to matter, and you can't read those ratings off the caps...
Installing openwrt without the serial cable
Re: Installing openwrt without the serial cable
Yeah, what I have done in the GC is doublesided tape a small USB hub inside it and use 3.3v into it - seems to work fine, and the sd reader I have hacked up has a 3.3v regulator which I have just removed from it and hardwired to the 3.3 of the router. detects cards and all, just no workie with the small flash.