seccomp is not enabled by default, it needs '-o drop_privileges' to be included in command line. A possible compromise for the meantime if this feature is not yet stable enough is to add a message post install if USE=seccomp letting the user know how to enable it.
See: * https://github.com/memcached/memcached/commit/ff68e8bc762b108ab0df59fdd6480699f0b75aff * https://github.com/memcached/memcached/wiki/ReleaseNotes159