you've got a very good shot at running p9p on it - p9p is
already known to run on some linux ppc platform. with
any luck, it should just work.

native plan9 will be some work. john points to the Blue
Gene kernel (which you can get somewhere, but as i
understand it isn't quite a "stock" plan9 kernel), and the
plan9 tree has support for some older ppc platforms.
those together should help to make it a comparatively
easy port, but it's still a port.

it'd be a fun project, and a nice port to have. not much
expandability, but a relatively cheap way to get a
multi-TB file server. my biggest questions to start would
be how the image is stored and what bootloader it's
currently using.

a