Fixed in git, thanks Christian.

New patch, moves motd to grml-live. Simplifies life.
(Removal of /etc/motd from grml-etc needs to be done, too, but not posting a patch 
for this...)

I've tried implementing a workaround, and it yields a working /etc/motd, but 
/etc/motd.tail is corrupted along the way. Don't understand why... Attaching 
patch for now.

initscript.postinst corrupts the grml-etc supplied /etc/motd.
Relevant code:
  if [ ! -f /etc/motd.tail ]
  then
    if [ -f /etc/motd ]
    then
      sed 1d /etc/motd > /etc/motd.tail
      [ -s /etc/motd.tail ] || rm -f /etc/motd.tail
    fi
  fi

This clearly does not check if initscript even owns /etc/motd. Sigh.
/var/run/motd is created later on in initscripts.postinst, but I suspect 
/var/run is not preserved ;-) (And should not be.)

The only option I see for preserving /etc/motd is to ship an /etc/motd.tail in 
grml-etc, possibly just being an exact copy of the current /etc/motd.

/etc/motd points to non-existing file /var/run/motd, mainly a cosmetic issue 
but should be investigated anyway.