In the latest git builds of tmux (with the latest terminfo db from ncurses), you can have clipboards propagate from remote tmux to local tmux (nested configuration) using the new OSC52 control sequence implemented in a patch from 2 weeks ago.

Also, if your terminal emulator (rxvt, xterm on Linux and mintty on Windows) support OSC52 escape sequences for clipboard setting, this means you don't need X11 forwarding anymore, since you can set your X11 clipboard from tmux.

If anyone can help me get in touch with the libvte people, I want to patch OSC 52 support in there as well, since this is a great feature that should _not_ be relying on X11 forwarding or whatever.