Partially because I didn't have a suitable micro-sized replacement, and partially because I wanted to be able to plug USB thumbdrives in the phone without having to cart an adapter around with me, I did this Frankeinsteinian arrangement. I took pictures of the whole process, but my camera promptly and helpfully corrupted the files, so I took a few pics of the completed job.





I soldered wires to the usual points on the motherboard (except ground, which I got from the mounting point of the deceased microUSB socket), and I let them outside from the old receptacle of the port; in order to do so I had to carve away a significant part of the top enclosure using a dremel.



I also removed a small part of the metal shield inside the top casing. I grabbed this picture online.



The red part is what I cut - there's a sort of "wall" that interferes with the passage of the wires from the board, and it had to go. I sanded the result so it wouldn't cut the wires and cause a short.

I then affixed the type-A USB socket to the phone itself using a large glop of melt glue; I did not have much available space to glue to, so I did the most of what I had - melt glue being the only type of glue that works better the more of it you apply. So far it seems stable; should the glue eventually let go I'll try two-part epoxy instead.

Then I just had to solder the four wires, and cover the solder points in more melt glue, and this is the result. Works perfectly for plugging in stuff, and all I had to do to connect the phone to the computer/charger was hack up a male-to-male USB cable. Coincidentally, if you ever need to do that, use chunky cables and keep them short; my first attempt with the cheapest cables I could find got the N900 complaining it wasn't getting enough power to charge.