Non-resettable zener/polyfuse for overvoltage protection
It sounds like you need a crowbar circuit. Essentially a circuit that will deliberately cause a short circuit should an overvoltage condition occur. The short should then blow a fuse.

But it sounds like you need high reliability in your application. With your current design, when the relay switches there will be a brief period with no output available. And the relay is a single point of failure.

Consider connecting both BECs to the 18V power supply, via individual fuses. have separate crowbar circuits on the output of each BEC. Then diodes to recombine the outputs together.

This allows the BECs to share the current during normal operation. And should one fail, there is zero transfer time for the other one to start supplying your 5V load.