How about copying text isolated in a pane to the system clipboard? For example if I have a split vertical pane, it will copy text from both panes.
Newer versions of tmux have an expand pane command mapped to Leader-z. It will grow the pane to full size, do your xopying then leader-z to toggle back to your normal split view.
If you don't mind trailing whitespace block select might be an option (option + select on OS X / ctrl + alt + select on Linux). Or just use copy mode and yank the selection to the system clipboard.