I'm not planning on supporting split panes (though I'm sure that feature will eventually be written), instead relying on multiplexers like tmux to fill that gap for the time being. I don't really know enough about tmux's control mode to understand how I might integrate it but I, personally, didn't like iTerm2's integration much so if I did interface with tmux at a lower level then it might look somewhat different to iTerm2.

That all said, at this point in time I'm not discounting any ideas.

Huh, ok. Split panes are one of the killer features of iTerm2 IMO, and also one of the reasons why tmux control mode is so great on it. :)

To each their own though, so if it doesn't work for you, then it doesn't!