Using stable branch rom I am able to boot Arch Linux from USB on my Thinkpad X200 without issue.
But using testing branch rom (grub_x200_8mb_libgfxinit_corebootfb_usqwerty.rom) gives me the options to boot Arch Linux, and then goes to black screen, stuck.
Yeah that's not a "bug" per se, since it's the same behaviour in Libreboot 20160907.
The "parse" option doesn't always work, so in this case you need to read the syslinux config files in your arch ISO and boot the arch setup manually. See:
Not a bug, so I'm closing this. This is a matter of providing user support. If you can't figure it out, try the SeaBIOS payload instead, which on most ROMs with GRUB is also available in the GRUB menu.