the developers howto says: Contents of an ebuild File 1. Variable Settings: ... ARCH This variable ... You can allow installation of these different types of packages through make.conf.
here is the rest of the comment: yet the stock make.conf file included in the stage 1 ppc 1.4_rc1 tarball (and possibly others) does not discuss how to enable this feature. there should be a section about ACCCEPT_KEYWORDS in the make.conf file.
yes, in the stock stage1 tarball there is an old make.conf. but once you get the system up and running and emerge -u portage (or emerge -u world), then etc-update, you'll get the new make.conf with this section.