Apparently back around 2006 there was an effort at Sun Labs to get OpenSolaris to work on CHRP(like) PowerPC machines. And according to the documentation, the kernel could even boot to shell on a G4 Apple.
That effort was called Polaris. It was difficult to find the CDDL-licensed sources, but I've made them available for everyone else to play with at https://github.com/andreiw/polaris
I haven't tried it out or done anything with the sources yet. The Solaris kernel is a pretty amazing piece of software, and a very portable and well-designed one to boot. I am glad Sun open-sourced it before folding, as it's code like this that should be influencing OS R&D for generations to come. It would be interesting to see the Polaris code being used as a base for an AArch64 investigation...