playing around with "Archipel" ( http://archipelproject.org/ ) which would depend on >=net-im/ejabberd-2.1.6[mod_admin_extra] (if such an ebuild for archipel would yet exist ;-) ), I've added the needed third-party module via IUSE.
The referenced patch attached adds this module to the source tree the same way "mod_statsdx" is implemented.
Created attachment 306411 [details, diff]
patch to current ~ARCH ebuild
Created attachment 306413 [details, diff]
two needed files fetched from http://svn.process-one.net/ejabberd-modules/mod_admin_extra/trunk/src/ as patch
ejabberd-2.1 is going to be removed from the tree as soon ejabberd-16.04 is stabilized. ejabberd-16.04 doesn't have mod_* flags any more.