Note: This no longer works, Gentoo and portage are noticeably smarter about this kind of thing now.
Yesterday, via the excellent Gentoo Bugzilla, I came across a reference to a tool for Gentoo I hadn't seen before, one that would have come in handy before. It came up because I was trying to emerge a new amarok, and the build was unable to find libpcreposix.la... the new libpcre doesn't include that file, but there was still a reference to it in something amarok needed.
The ebuild is called dev-util/lafilefixer, and to run it:
lafilefixer --justfixit
Whoever put this together has now helped me a great deal... I had literally hundreds of .la files that it updated. Plus it fixed amarok and let me complete my emerge update.