Libreboot and grub

Since you have Libreboot installed your computer first boots grub that's located on the flash-chip of your X200. That grub will look for its configuration in a file called grub.cfg. That file is also located in the flash chip, in a file system called "CBFS".

In the menu-entries of the grub bootloader you will see something like "look for grub.cfg outside of CBFS". Using that option would normally look for grub.cfg on your hard disks and allow you to boot using that. In this case modifying the file on your hard disk is an easy way to modify grub's configurations.

However since you have full-disk encryption, that menu entry will not find anything. That means that you have to modify the configuration file located in the flash chip. That means you should read Option B from the links you posted.

After you know how to modify the grub in the libreboot rom and flash it, you should have two options:
1) modify the default boot entry to decrypt the hard disk and start the linux kernel
2) modify the "search for grub.cfg..." entry to decrypt the hard disk and then search for grub.cfg from it.

The latter option gives you more freedom to experiment with different grub configurations without having to reflash each time. However if the only thing you want to do is to decrypt your disk and boot, then I suggest 1). For that, follow the last step at http://libreboot.org/docs/gnulinux/encrypted_trisquel.html on how to create a suitable grub.cfg.

PS: a third and possibly quite easy option came to my mind. If you want to experiment without reflashing yet you could try this:
3) the entry "look for grub.cfg outside of CBFS" will also look for grub.cfg from usb flash drives. So you could write a grub.cfg on a usb drive, insert that into your computer when it boots, and choose the "look for grub.cfg outside of CBFS" option.

Thank you for your answer, after all I've decided to be radical and do as libreboot suggested and re-flashed libreboot with edited grub.cfg as explained.