How could I bind to last (most recent) session? The obvious
last-session leads to an "unknown command" error.
bind-key z last-session
Don't know about how to bind to the Eject key (it's too far from the home keys anyway!) but this will do the trick:
bind-key -n z switch-client -l
In recent versions of tmux (e.g. 2.6) the key
L is already bound to the command
switch-client -l which as user47464 pointed out is probably what you want.
You can check your bindings with
... bind-key -T prefix L switch-client -l ...