How could the system interlock masters automatically? How would the system know that your pump was out of service? I suppose if you have a flow meter, and use a IFTTT (If This Then That) service, you could monitor water flow and If no water flow, then disable s1 and enable s2? But I have no clue how to implement that or if it is even possible. This would be terribly complicated. Perhaps if you know you have a failure, you could manually do the following until a repair is made.
Each zone gets none, one or two masters assigned to it. If s1 is your pump supply (set as Master Valve 1) and s2 is city water (not set to master since you don’t normally us it), and your pump fails, just go into “Edit Options”, “Configure Master – Master Station 1” and change if from s1 to s2. Now all zones will use s2 (city water) as their water source until you change it. When you fix your pump, go back into “Configure Master – Master Station 1” and change if from s2 to s1 and all zones will use pump supply water. This way your wife won’t have to remember anything and all changes are done programmatically which can be remotely. No need to physically visit the controller to make physical changes like removing or switching wires. Just make sure you don’t program s2 with a time thinking it is a sprinkler zone.