If you have to protect against overvoltage, you will probably also have to protect against undervoltage.
Most (all?) CMOS chips have diodes from each input to both rails, to provide electrostatic protection. These are silicon diodes. If you shunt them with low-capacitance Schottky diodes, essentially all of the current due to overvoltage or undervoltage will flow through the Schottky devices. The impedance of your source will probably limit the current to a safe level for the external diodes.
The problem with this type of overvoltage protection is that most regulators will not sink current. Repeated overvoltage may raise the supply pin(s) to a level which will damage either the ADC, or the regulator, or both. You need to provide a path to ground for current that flows through the protection diode. A low-value resistor (100 ohms?) will probably work, but it will waste some power. An active sinking supply such as a PNP emitter follower will work as a supply to clip to, but it takes up space.
Undervoltage protection can probably be accomplished with a low-capacitance Schottky diode to ground.