The real problem with many chromebooks is how they use firmware blobs Intel provides as injectors in Coreboot. They aren't really open firmware at all.
AMD is pretty much the only company doing anything in this regard. Many of their recent chipsets are supported, like fm1 and fm2.
There's a fork of it that is 100% free software: http://libreboot.org/ . Unfortunately it only supports one computer, the Thinkpad X60.
(They even removed the CPU microcode updates, which IMHO is going a bit too far; there's already microcode in the CPU, and Intel issues those updates to fix various errata in the hardware. Maybe it was done more as an ideological thing.)
And of course having different firmware for different OSes defeats the point of firmware standards. I think it is possible to run UEFI as a payload in coreboot.
AMD is pretty much the only company doing anything in this regard. Many of their recent chipsets are supported, like fm1 and fm2.