So first extract grubtest.cfg from the ROM, modify it (see right above),
Then put it back inside the ROM, and flash your modified ROM.
See here for instructions, rather straightforward:
https://libreboot.org/docs/gnulinux/grub_cbfs.html
When booting, it will fail because by default, it reads grub.cfg, which we
haven't modified yet. get back to the GRUB menu, and select grubtest.cfg (3rd
or 4th option maybe). It should work.
If it does, finish the job with the "final section" in that same page. Make
sure to rename the sections grub into grubtest inside grub.cfg that was
generated (just read the instructions, nothing special).
Final touch, you can remove (replacing it didn't work for me) the background
image, so that you have a black background instead:
https://libreboot.org/docs/grub/index.html
Make sure that the name is the same as the one in the command. For this, use
the print command to display what's inside the ROM. When you're done, flash
the ROM one last time.
It seems you extracted grubtest.cfg from the Libreboot ROM.
And that you're stuck with modifying grubtest.cfg because you don't find the
"Load Operating System" section.
Inside, you have "Load Operating System" followed by brackets {} with a bunch
of stuff related to booting an OS.
Erase what's inside those brackets only, and paste in or write down your
cryptomount commands that allowed you to manually boot.
Start here.
