Hi Stuart

>> what's the easiest way to safely limit typical trigger/gate voltages
>> to 4.5v before going to an ic's inputs
> zener diode (perhaps plus buffer-opamp)

I forgot: do you have +/-12V available in this circuit If yes, then use
a comparator which makes a defined on/off voltage (close to power), and
then reduce it with two resistors setup as potentiometer and a buffer
after that.

Florian