Almost all mirrors remove the old version when a new version of chrome appears. This mirror also has the old(er) versions: http://mirror.pcbeta.com/google/chrome/deb/pool/main/g/google-chrome-beta/ Maybe it could be included as a secondary source URI in case the main one removed its version already?
Mike added a new ebuild: https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=f1ae740058f83dae151f53f7f4c4a7821ed0d708