A suggestion for portage/emerge. When emerge is run with the -uD flags, hence providing the ability to break reverse dependencies, a notice should be provided at the end of a build session saying that revdep-rebuild should be run, in case reverse dependencies were broken. A usability suggestion more than anything else.
No, it couldn't. That's done by ebuilds completely outside portage's knowledge. What you really want is http://www.gentoo.org/proj/en/glep/glep-0042.html