Hello,
The thing about using LEDs as zeners is that the forward voltage may vary too much. You could test some though and see what you get. Test over temperature too though if you expect a temperature variation.
Another way is to use a shunt regulator. A shunt regulator keeps the voltage at a node constant by shunting current away from the node. Schottky diodes can then be used to wire 'or' all your inputs to the shunt regulator node, thus keeping each input clamped to a level close to the shunt voltage plus a little diode drop. The diode voltage variation will be less than an LED voltage variation and more predictable too, and also less than a low voltage zener like 3.3v because low voltage zeners often have a wider voltage variation also. You should carefully test anything you use though before you actually rely on it to protect anything valuable.