You may already know, but in case not, you can also use this tmux feature from your editor. For example in vim

  :w !tmux load-buffer -

Writes whatever you have selected in vim into the tmux buffer through standard in.

Elsewhere you can paste like pbpaste.

  :r !tmux save-buffer -

The save-buffer command is’t the most intuitive name, but it’s saving to a file, stdout in this case.