Turns out that the GENERIC 1.0 config-file has no option for the
UNION filesystem - I had to add it myself.

Also I believe the man page for mount_union is not correct, as I still
had no success to mount a rw union. 

I did as described in the manual:

mount -t cd9660 -o ro /dev/cd0a /cdrom
mount -t union /usr/cdrom /cdrom

Yields into a ro /usr/cdrom, which is not what I want, as I would
like to write there. the option -o rw is ignored and still yields into
a ro mount.


-- 
Markus Illenseer