I think that if we want to promote this computer as "respects your freedom" we should be able to run one of these distributions : http://www.gnu.org/distros/free-distros.html
Unfortunately none of them is available on PPC yet.
We may try to contact them.
Depends what free is. I don`t find Debian or Ubuntu "unfree", even there are some teams and entities behind.
Well, getting it work on Debian is good enough. Some nice add ons would be welcomed.
On by-specs similar PPC64 home computer (A1 X1000) I really liked MINT PPC concept (backport of Mint LXDE to Debian - fast and usable OS)
http://mintppc.org/aboutSadly, it has no releases for 4 years now.
Besides Debian and classic Ubuntu - OpenSUSE, Fedora and Lubuntu exist in PPC32/PPC64 versions.
Surprisingly, they all have options to add (if not included) quite latest Firefox, Chromium, Libre Office, GIMP ... GUIs (LXDE/GNOME ...)
with recent addition of MATE. So, the state of Linux for PPC is not perfect, but is still good and very usable.