Users should be able to obtain a list of installed sets (ie. the contents of /var/lib/portage/world_sets) via emerge. I would suggest an option "--installed-sets" that lists all the installed sets (in the same way that "--list-sets" lists all of the available sets)
It's fixed in git to show them in emerge --info: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=44809c14de087f87ee0fa9c5a21ec94e4787c3ce
This is fixed in 2.2.0_alpha20.