here is a much better idea that doesn't even need an extra switch!
attatch a SPDT relay to the computer power supply, wire the NO of any button on your panel to the common of the relay.
Wire the NC of the relay to the motherboard.
Wire the NO of the relay to the keyboard encoder.
What this does is causes a button on your panel to turn on the computer, but once it is on, it will run to the keyboard encoder, not to the computer power.
To turn it off, just shutdown through your OS.
(If this doesn't make sense, I can post a circuit diagram)
(oh, and no I did not do this on my cab. Mine is hard powered, so I mounted a lightswitch underneath to the front right)