http://www.bluefi.co.uk/story.php?news=7
Sony DVP-CX875P 301 Disc Progresssive DVD Changer
Can get it as cheap as US$470.
Unfortunately only available in US, Im in Australia....
Would love to import one at some stage.
This unit works with other (commercial = evil) htpc software.
Not sure if it has RS232, dont know how they control it.
One option is to get an Infrared Unit, then you just learn the codes into the computer and shoot them out for each disc.
E.g. You keep the girder command sequence in the movies.XML file so a disc would have the commands (Disc Button) (1 buttton) (0 button) (4 button) for disc 104
Should work.