I've been meaning to do something with this for a really log time.
Anyway, at

      ftp://ftp.netbsd.org/pub/NetBSD/misc/danw/

you'll find the sources for a program called "mdisk", which you can
use to edit Mac partition maps from NetBSD, to the point of being able
to have both NetBSD and MacOS on a disk together. (Note that since it
writes the partition map out directly when you save it, the kernel
won't notice until you reboot.)

Someone should take this and do something useful with it.

-- Dan