Debian unstable has mysql-5.5.24 working. As of Gentoo mask of mysql-5.5.22 a new version is needed?
mysql-5.5.27 is in the official overlay. You can use layman to get it. The devs should move it over when they get time. Meanwhile, we are looking for people to test the overlay for 5.5 series and report to bug 338818.
I'm closing this bug as version 5.5.28 is already in the overlay. If you want to track the unmask of mysql/mariadb 5.5, please follow bug 413883.