After a bit of head scratching and further reading around, I am please to
report that my MacBook has now been Librebooted.
In terms of getting a USB OS to boot under the stock Apple EFI firmware, my
understanding is that a 32-bit EFI compatible distro is required. I believe
most 32-bit distros are BIOS only, but please feel free to correct me. The
one used in the end was SparkyLinux (based on Debian).
A further change was requird to add the following into the Grub configuration
once installed (described in the Libreboot Wiki):
GRUB_CMDLINE_LINUX="iomem=relaxed"
Flashing then as per the Libreboot instructions worked perfectly, and is now
happily running alongside Trisquel. This is now the second bit of hardware
that can be considered 'fully free' alongside my LibreCMC'd Router, so I am
quite pleased. :)
Thanks for the responses, and hopefully this could help anyone else in the
future.
