The usual way of protecting I/O pins with narrow margins (and pin injection currents in the order of 5mA usually) is with a resistor and perhaps an external clamp schottky:
simulate this circuit – Schematic created using CircuitLab
The reason for the 0.3V is usually that there is a schottky internally, but it cannot handle much current; adding one (or two for bidirectional protection) that can handle the current due to a voltage excursion event is a simple and effective means of protecting the pins. The resistor limits the current in the schottky and therefore maintains \$V_f\$ at a nice low voltage.
TVS devices have wide variations of turn-on voltage, full clamping voltage and those parameters are also thermally dependent, making them unsuitable for protecting I/O pins in this particular case.
In the case of I/O pins with schottky devices and a rated pin injetion current (usually the maximum sustained current the internal schottky can handle) a simple resistor can suffice if the maximum voltage can be defined.
Typical arrays for mult-line protection can be found at some manufacturers
Note that if you expect a high voltage (too fast even for a front end TVS as JonRB alludes to - some have slow reaction times) you may want to use pulse-withstanding resistors. These maintain their resistance even after a large transient event; standard resistors may not.