Okay, we did some experimenting:
- the error message is "onleesbaar" (unreadable)
- by playing with /usr/sbin/osso-usb-mass-storage-enable.sh and especially with the g_file_storage module and it's options (rmmod and modprobe are our friend), we managed to present the following over usb:
1) 2 devicefiles, one for the sdcard (doing nothing) and one for the vfat partition (this is the default way) -> same error
2) 1 devicefile, one for the vfat partition -> same error
3) 1 devicefile, one for the complete flash storage with mbr (/dev/mmcblk0) -> same error
4) 2 devicefiles, one for the sdcard (doing nothing) and one for the complete flash storage with mbr (/dev/mmcblk0) -> same error

From the manual of the car we can read there are the following limitations:
max 500 directories
max 64000 files
min 256 MB of size
support for playback of mp3, wma, and wav files. other files will just be ignored.

We will do further experiments like copying the entire vfat contents to a usb stick, trying to clean up the special dotfiles, etc. *sigh*