I had a similar issue on my Zotac 330 Ion. I couldn't use sudo on the MD because sambahelper told me I had the wrong password. To get around this I had to set an actual root password for the MD.
When you log into the MD your prompt should tell you what moon# the MD is... remember the number.
On the core/hybrid you need to open a terminal in KDE or press Ctrl-Alt-F1 to get tty1. Login with your username and password. Then to become superuser type:
$ sudo su -
and enter your password. Change directory to /usr/pluto/diskless and chroot into the MDs structure. Once you have done this you can change the root password for the MD with the passwd command.
# cd /usr/pluto/diskless
# chroot XX <-- replace XX with the moon number (device number) of your MD.
Enter new UNIX password:
Enter the root password for the MD and then exit the chroot environment and give up superuser by typing 'exit' twice.
Now you can login as root on the MD with the password that you just set and you will be able to edit your xorg.conf file.