I'm unsure if this is helpful for anyone, but I started hacking on yaboot a few days ago for GCC10 and better ext4 support. This boots my PowerMacG5, but I have yet to try booting a 64-bit ext4 /boot partition - the updated e2fsprogs should support it though. It's still a little kludgy, but it's here:
https://github.com/rsaxvc/yaboot4 -Richard
