I hate to say it, but as great as the PowerPC arch is, it requires royalty payments, etc.; at least according to this:
https://en.wikipedia.org/wiki/Comparison_of_instruction_set_architectures.
The only truly open architectures are: RISC-V, OpenSparc, and OpenRisc. Perhaps this is one of those situations where a certain version of the instruction set is open and later versions are not (ex: ARM).
RISC-V adheres to the open source philosophy very well and there are good FPGA implementations for testing. The main page of this project explains that PowerPC was chosen because it is newer
and cleaner; well, RISC-V is one of the newest architectures.
From what I understand, the PowerPC chip that this project is targeting is:
1) Has uncertainty as to whether the manufacturer will continue or not.
2) Varies from the PowerPC CPU targeted in the official Debian PowerPC port.
Debian is currently working on a RISC-V port that would be maintained by them.
Strangely, I don't see any comparison between OpenSparc and PPC on the main page. I would think that OpenSparc would have similar advantages to the x86 as does the PPC. Unfortunately, SPARC is no longer maintained by Debian.