What do we have to know in order to start on this? How much do we already know?

We (the collective "we") already know everything we need to know to write our own player app for the car player. All of the hardware I/O is well known.

As far as the file system is concerned, we also know pretty well how the FIDs are stored. We might have to build our own database mechanism and our own method of synchronization (perhaps we'd choose to use Hijack's FTP daemon instead of the emplode protocol, or perhaps we'd try to make it emplode-compatible, who knows?), but it's do-able.

Then again, if the idea is to really make our own player app, perhaps we could just forget the existing file system and write our own. Maybe just store the MP3s raw along with M3U files and have our player app read them directly. Of course, the empeg guys deliberately did it differently than that because they wanted a low-overhead database instead of a read-everything-in-real-time file system, but for a start, it might be easier to try it as straight files.