I'm sure that it is theoretically possible but I don't know if a modern version could be built without code changes or not. I also haven't heard of anybody building a more modern version for PowerPC (on OSX anyway) and I'm sure if it were trivial it would have been done. At any rate, it is not a rabbit hole i have been tempted to dive into :).
I did find these instructions: http://www.spy-hill.net/myers/help/boinc/boinc-on-darwin.html
...but that's from 2009.