tmux "last-session" binding

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 Prefix-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 Prefix-?:

...
bind-key    -T prefix       L                 switch-client -l
...