My first choice is still a networked terminal server. For people who don't already have a hub in their car, just a crossover cable would do. But it still means modifying the sled so that the ethernet port is available when docked.

The part that I couldn't get past (ok, it's been >6 mo since I checked) is that none of the terminal servers come with driver source code or linux binaries. The way I'd want to use one is to have it plug in to the network and have special device files that map over the net to the serial ports on the terminal server. The huge advantage of doing it this way is that you don't need different software for people with a terminal server than for people using the built-in serial port (just a config option to specify which /dev/tty file to use). But without drivers, it's a bit more difficult.

Quite some time ago, someone posted a link to a mini terminal server that came with instructions on writing your own TCP code to access it over the network. If someone (or a team) wanted to write code for that, then we could all buy the same unit and have a working solution.

I don't (yet) have need for another serial port, but with the progress of the GPS project and the news about the upcoming release of OBD-II codes, I'm sure I'll want more serial ports eventually.