The current version will reflow. When I use it through byobu I sometimes have to initiate reflow with ctrl-a, shift-F.
It may not reflow as expected if you have more than one client connected to the same session - since it limits itself to the smallest attached client. If you leave your ssh session connected to the session and then connect again locally it is not obvious why reflow won't work, but you may need to just disconnect the other client.
1.8 Changelog: http://sourceforge.net/p/tmux/tmux-code/ci/master/tree/CHANG...