New AWS regions are using an updated authentication mechanism which is only supported by recent versions of s3cmd, current stable version is 1.5.2. Tested locally by just renaming s3cmd-1.5.0_rc1.ebuild, the resulting s3cmd tools works just fine for me.
+*s3cmd-1.5.2 (10 Mar 2015) + + 10 Mar 2015; Patrice Clement <monsieurp@gentoo.org> +s3cmd-1.5.2.ebuild: + Version bump, EAPI bump and conversion to distutils-r1. Fix bug 542786. + I couldn't help but convert it to distutils-r1 and bump its EAPI (distutils has been deprecated).