Yes, I'm sure you can Charlieplex a keyboard. If you can turn on a single LED that way, you can select a diode/switch pair in exactly the same way. Thought experiment - Charlieplex a bunch of LEDs. Now put a switch in series with each LED. When you select a given LED, current will flow if and only if the switch is closed. Now measure the impedance across the two select lines you used to select that LED. But the driving circuit has to be reconfigured constantly to allow for the bidirectional polarity of opposing diodes.