See recent thread in gentoo-dev: http://thread.gmane.org/gmane.linux.gentoo.cvs/729967/focus=93592 eix misses a feature that would allow to show (or query) fields like "longdescription" that appear in metadata.xml.
The eix database contains only metadata (i.e. the information stored in metadata/ directories). The way eix obtains this data (e.g. reading directly from metadata/ or from /var/cache/edb/ (e.g. from the sqlite database if portage uses sqlite backend), reading from another eix database etc.) does not require/guarantee that metadata.xml can be read. Thus, it would need a complete restructure of eix and the way how eix works to read/parse metadata.xml (not to speak that eix currently contains no xml parser). I don't think that it is reasonable to extend eix in such a way.
*** Bug 572662 has been marked as a duplicate of this bug. ***
*** Bug 583990 has been marked as a duplicate of this bug. ***