Ok, I think I have figured this out now. You don't need to modify buildx itself, but if your docker host is not ppc64 you need to run this command line first:
docker run --rm --privileged aptman/qus -s -- -p ppc64 Then you can make linux/ppc64 images with buildx (tested on arm64). Note that Pike 8.0.1116 is no longer available in sid, but 8.0.1738 is there.
