http://wiki.maemo.org/Compiling_the_kernel has some info, and I think I've seen info from people who've made cables posted on itT a couple times.

However, on still more thought, I believe qwerty12 has posted an initfs that used either a USB-RS232 cable and USB host mode, or made the N8x0 appear as a USB-RS232 in USB device mode (I don't recall which) for serial console use at bootup; reflashing the initfs with such an image would be as easy and use only COTS hardware...