I played around a little with it today, and discovered something by accident. The addon has been spawning the 'mlb' player process reliably, but nothing plays. During one of my attempts, I selected the stream, and the file began to be created. Then I ssh'ed into my XBMC box and killed the 'mlb' process. The file then began to play. This makes me think XBMC is waiting for the 'mlb' process to exit before playing the media item.
Another observation: I have not been able to play a NexDef file longer than 9-10 minutes without the player dying. I've tried it with XBMC and mplayer. It's not a buffer issue, because I've given it a 10-minute head start. I also don't think it's a corrupted file, because the next time I play the same file, it will get past the point it had previously bailed. So even using the exact same file, mplayer will bail at different, inconsistent points. Here is the mplayer output for one of these files, and the message it produces when it bails:
http://pastebin.com/D9PziECH
XBMC does the same thing, but without as much detail. Anyone know what this is all about?