http://archives.gentoo.org/gentoo-dev/msg_3e335b01696e786598c55b6ace346beb.xml repoman should check the value for <herd> against herds listed in herds.xml
Progress over here: http://archives.gentoo.org/gentoo-portage-dev/msg_a8b8d25c2f0dbbbce32ef311f1bff216.xml
Patch has been accepted, closing. http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commitdiff;h=7c11c8ec5f568984ee720fc677959f9d2c9dfd32