A new eclass will simplify multiple ebuilds that depend on dev-util/xml-core since duplicate funcationlity will not be required. I propose that the xml-catalog.eclass work the same way as sgml-catalog.eclass and uses /usr/sbin/update-xmlcatalog provided by dev-util/xml-core to manage the catalogs.
Since dev-libs/libxml2 already provides the functionality to manage xml catalogs with the xmlcatalog command, I propose to drop the idea to implement an eclass with update-xmlcatalog and xml-core but rather implement an eclass around xmlcatalog from libxml2 instead. There are already a few eclasses in portage that could take advantage of this eclass. xml-core will be another ebuild that could take advantage of an eclass built around the xmlcatalog command. Here are a few ebuilds in portage as an example: