I guess it all depends on the documentation. Sun's is excellent e.g.:
http://www.bitsavers.org/pdf/sun/sun3/Sun-3_Customer_Mainten... - on the Sun 3/60 you have a rather nice boot ROM which does similar things to UEFI but without the drama. It will let you netboot it over tftp, for example.
Which sort of PPC machine is it? Have you already managed to get e.g. NetBSD running on it?