Gregory's ServerOpenBSD is completely open source, but not by strict FSF rules. If you want that, you can turn an existing OpenBSD system into LibertyBSD using scripts to build a new kernel, and remove offending stuff from ports.
I would recommend it only for releases, doing this on snapshots would get tedious unless you just updated -current like once a month.
Plus they haven't touched this for a couple years so it may need patching, not sure. I may test it and make some content about it.
Now if you wanted to boot OpenBSD from a Core/Libreboot system, you should use either SeaBios or Tianocore as the payload. The Grub kopenbsd module is broken and doesn't properly initialize hardware. At the very least chainload to OpenBSD's provided EFI firmware to boot.
@PublicLewdness
I would get behind the Hyperbola project if they intended to soft fork OpenBSD and go the LibertyBSD route instead of a hard fork.
A hard fork is a dead end if you ask me and an engineering nightmare, given the seriousness of OpenBSD development, their existing kernel (hyperbola) is already woefully behind upstream. Even if they pulled it off, it would be security wise the same as running old OpenBSD versions from years ago.