Additional overvoltage protection after RBO 40-40 with Zener diode
What you propose is a lower voltage (20V) Zener in parallel with the internal Transil (40V), it will work up to 20V but common zeners aren't designed for transient protection.
I use the RBO40-40 before the electronic module voltage regulator, and another TVS in the output of this regulator, matched with it's output voltage.
The RBO40-40 is designed to protect electronics acording IEC/ISO standards, maybe you can use a high input voltage regulator and the TVS in it's output.