There is one workaround: start your system and wait until it boots up. Press ctrl+alt+F1 to switch to a text console. Log in with your user, and after using sudo, use your favorite editor to edit the xorg.conf.
sudo nano /etc/x11/xorg.conf
About the other question regarding why you can't edit the file from the live cd: there can be more reasons you cannot edit your installed system's files.
Some of these reasons can be:
- When you boot from the live cd the user that the installer uses when launching a terminal is not root. Therfore you have to execute "sudo -s" at the terminal window to become root so you can edit the file on the already mounted FS.
- The FS (filesystem) that contains your installed OS is mounted as read-only. In this case you have to remount it as rw.
To detect if a FS is mounted as read-only:
sudo mount /mnt/mount_point | grep ro
If this command returns a line then the mounted FS is mounted as read-only.
To remount a read-only mounted FS as writable:
sudo mount /dev/block_device_id /mnt/ -o remount,rw