Need for a programmable keyboard: It would be nice to select the KVM mux via a single keypress of unused keys on the keyboard.

Programmable keyboards aint necessarily cheap, and there are at least fewer of them, making selection of one that suits your needs harder.

The PS/2 protocol isn't terribly complicated and is easily connected (PS/2 clock to PIC "int" and something else to data).

It's cheap:

• PS/2 extension cable (eg. male and female connectors) ($2.50)
• 16F628 microcontroller ($2.20)
• enclosure ($1.15)
• protoboard ($2.00) (though considering it's just power connectors, two clocks, and two datas, a board may not be needed...)
• probly a cap or two and a few other odd parts...