Very low power shunt regulator for signal clipping
R1 would be around 200 kOhm.


If R1 is 200 kohm and the worst case over-voltage is 30V then the worst case over-current into the unprotected input of BUF1 is: -

(30 volts - 3 volts) / 200k = 135 uA.

If BUF1 has a spec that tells you the maximum input current I'd make a small bet that it's several times the figure I've just calculated.

If this is true then R1 is all you need for protecting the input.

The ALD111933 won't do what you think it should - protection levels of 10V might be obtainable. For a start it doesn't start switching on until the gate is at 3.3 volts then it's only guaranteed to conduct 1 uA. With a gate voltage of 7.3 volts Id is specified at 3mA. Do you see the problem of this type of circuit?