pam authentication is available in ejabberd-2.0.0. It is enabled by --enable-pam. The current ebuild does not specify anything about whether to use pam or not. Simply adding a pam use flag with $(use_enable pam) did the trick for me. Reproducible: Always
Another issue with pam, I don't know if this is fixable with the ebuild, is the permissions of /usr/lib/erlang/lib/ejabberd-2.0.0/priv/lib/epam. This is a helper binary to perform the pam authentication, therefore it seems to be owned by root and have to have +s permissions.
added pam and permission modifications. Thanks for hte report.