The way tap water works in my house is that there's a cold water tank in the attic which the mains fills via a ballcock. There's no way it can get from that tank back into the mains even if the mains is switched to "suck".

I wonder if it would be feasible to just replace that tank with a bigger one? Presumably bigger houses have bigger tanks so a range would be available. You'd need to make sure the floor was strong enough. Water is heavy.

It might be worth wiring up a sensor that could tell you if the water level dropped, and/or the mains was cut off, so you knew when you were using your reserve.