I just bought a 3Com mini-PCI card for this purpose, but I have not "put it in" yet.
What I plan to do, what I've seen done to other mini-PCI cards, is solder wires right onto the mini-PCI card. There really are only 4 needed RX-+ and TX-+, that's it! The mini-PCI type 3 cards spec a standard connector on the board for the RJ45 dongle/cable to plug to. But you can just wire directly to the pads on the little board, and then put a gob of glue stuff over it to make it permanent. You can either have the cable sticking out of the box, or bring it to a real RJ45 that you fashion to fit (my machine is a modem version, so I have to decide on how to do this.