Not a finished product yet, but after some evenings of coding I've got the following framework :

Server side :
- middleware software based on openldap server.
- multiple backends possible. Support for multiple empeg carplayers as MP3-source is functioning.
- internet streaming
- refrences to other LDAP-servers ( share music with friends/neighbours )
- Access Control based on LDAP authentication.
- Unlimited number of tunes/playlists possible

Player side :
- Hierarchical menu structure, as seen on the empeg carplayer.
- LDAP-queries can be used to generate dynamic playlists
- Uses existing font files, including all icons

As I said before, it is not functional at the moment. Everything is into place except for the actual decoding and playing of tunes.
Here's a simple schema explaining the intended behaviour :
http://fvgestel.dyndns.org/receiver/ldaprec/ldapreceiver.htm

The current 3rd party players are quite nice, but they still are very restrictive in selecting music. when using 300 playlists it's a pain to scroll through all entries to get the desired entry.

Got to go now. Will post update later