I have no WL-550gE so I can't try this...
The internal pictures clearly show (filled) solder holes for a 2x4 pin header marked with +5V, D1-, D2+ and GND (and D2-, D2+, of course). So just connect your usb device there (in my opinion, additional current limiting/filtering on the data lines isn't necessary; maybe you want to include a fuse between +5v and your device) and it will most probably work. Standard usb wire colors should be: red for +5V, white for D-, green for D+ und black for GND. Just in case you need them.
Bear in mind that neither ASUS nor this forum nor I are responsible for any damage, so use common sense and only try this if you can afford to lose your router...