Renaming oracle-jre-bin-1.8.0.11.ebuild to oracle-jre-bin-1.8.0.20.ebuild works for bumping oracle-jre-bin in my ~amd64 box.
+*oracle-jre-bin-1.8.0.20 (02 Sep 2014) + + 02 Sep 2014; Sebastian Pipping <sping@gentoo.org> + +oracle-jre-bin-1.8.0.20.ebuild: + Dare to bump to 1.8.0.20 myself (bug #521304) +