@faidon pointed out on irc that he had seen log events on fluorine with dates in timezones other than UTC. I poked around a bit in the Monolog source and found that this is feature added in Monolog 1.12.0. Each log event gets a DateTime object that eventually is formatted to a string via DateTime::format(). Since Monolog 1.12.0 these events are explicitly cast into the timezone returned by date_default_timezone_get().
At this point I'm not exactly sure where in the wiki startup cycle we use $wgLocaltimezone to set the timezone, but it certainly happens:
terbium:~ bd808$ mwscript eval.php enwiki > echo date_default_timezone_get(); UTC terbium:~ bd808$ mwscript eval.php barwiki > echo date_default_timezone_get(); Europe/Berlin
We need to find a way to control the behavior of Monolog to allow us to force the log event DateTimes into UTC.