On the wiki page for the prototype of the MB-6582 (http://www.midibox.org/dokuwiki/doku.php?id=wilba_mb_6582_prototype) it says: "Expansion port: In case I ever get around to using external filters or external analog inputs (joysticks, ribbon controller) or whatever else I can dream up, I can wire it through this 25 pin sub-D. I think I’ll make it female like a printer port." Hope that explains it. :)
I've been lurking for a while, I made an account a bit ago as it seemed useful for group buys and the like. As for the PCBs, I'm all set, I've ordered the base and control panel ones, as well as a bunch of SIDs. ;D