There are multiple problems with tmux on OS X. pbcopy and pbpaste, for instance, are entirely unusable. Even if tmux has some nice benefits over screen, it probably isn't worth it to the Mac user to switch.
tmux actually has a way to work around this: whenever you reattach, it looks at a whitelist of environment variables representing external resources, and either updates its own environment with the new values, or removes those settings from its environment if the place you're attaching from doesn't have them available (naturally this doesn't magically fix any processes already running inside tmux, but every new window you create will be set up correctly - unlike screen, where you have to kill every window and end the session). By defaut, this whitelist includes variables used for X11 authentication and the SSH agent. If OS X has a similar environment-variable-based connection system, you might be able to add it to tmux to get things working.
I have a wrapper script that reproduces tmux's behavior of automatically updating a set of environment variables. It goes a step further and also updates them in your running bash shells. For this to work you have to register a signal handler in your .bashrc that sources a file with the new environment. The script is available at https://github.com/sciurus/splatbang/blob/master/rescreen
https://github.com/ChrisJohnsen/tmux-MacOSX-pasteboard
The documentation recommends changing tmux's default command, but if that weirds you out, a simple alias to "reattach-to-user-namespace pbcopy" also works.
Would AppleScript do it and would that work in any application?