Since it is Core boot and all open source drivers, I imagine it won't be hard. However given it's just started shipping, I don't think anybody have much experience with it yet.
It is interesting how a community that's probably installed their own software on their hardware for the last 20 years, suddently care so much about if it ships with one or another open source operating system.
Coreboot isn't the issue. You need a payload for Coreboot. For x86 architecture, people use Seabios, but that won't work with the Pixel C, because it's ARM based.
Some people suggest you might be able to get Tianocore working, but I've no experience with that (I'm running Xubuntu on a modified HP Chromebook 14). ArchLinux has a Samsung Chromebook guide - https://wiki.archlinux.org/index.php/Samsung_Chromebook_(ARM.... So it might still be possible with a lot of effort.
Incidentally about your closing comment - when PCs were towers it was a simpler matter to install your own operating system and make sure you choose compatible parts. It becomes a lot more difficult when they're pre-packaged laptops - you can't really build one yourself. Google doesn't want you to switch away from ChromeOS because their business model centres around their apps (Google Drive, etc.) and ads for their searches. So that's why people have started caring about whether it can run a certain OS easily.
