Hiya,
Hurray, after some trial and error I finally got this one (took long enough):
CODE
START /W WMP6CDCS.EXE /C:"SETUP_WM.EXE /Q:U /R:N" /Q:U /R:N
(Change the :U to :A to remove progress bars.)
Trick is, both installers need to be silenced in order for this to work unattended.
The /C: switch passes on commands to the embedded installer (now that I look back, it seems lots of MS stuff is using this method). Live and learn.
Good Luck!