I've found several articles dealing with repairing the usb on the Nokia N900, nothing on the N810. Seems these things are surface mounted and difficult or impossible to replace. Given that you cannot flash without a cable to pc, it may be a no go. I will probably disassemble it anyway to see if I could swap the motherboard from the good (but not as shiny) one.

You can see that I have absolutely too much free time.

This was interesting, but I have no idea about the corresponding points on the N810.

http://www.quanttrom.com/nokia-n900-usb-port/