+ permal
I meant to say that you can cater for different voltages just by adding another resistor. But it does mean that you would have to change the resistor to suit the voltage range that you are using, as shown in the attached schematic below:
Yeah, but in my case I have 5V and 12V available and I don't know before hand what will be connected so I'll have to go for the zener-variant.
Sure 
-- great explanation by Spec here --
Thank you

Ok, so I'm now at the following schematic.
Questions:
1. I've noticed that you don't add the pull-down resistor in your example schematics, @Spec - is there a reason for that? The MCP23017 do not have built-in pull downs so I see no way of excluding them (R1605).
2. R1603 and D1601; the current limiter and LED, how should I reason about those? I'm sure they affect the circuit somehow (a part from drawing current).
3. If the wire between CTRL and R1601 is "long", would a decoupler cap do any good? Sprinkle a 10nF on it?
4. The inrush resistors - how do I calculate appropriate values for them? If they can be reduced to 10R, then they feel somewhat arbitrary to me (though I assume they are not)?
5. Am I reading the data sheet for
CZRQR52C6V8-HF correctly, in that its foot print is SOD-923F, equal to 0402 (1005 metric)?