But it also sounds like a dream if it could actually work. If you have enough local, performant disk that you are sharing with the cluster you should be able to get good performance and rely on the system to provide resilience and extra space.
In practice you can't get high-availability this way without additional logic and circuit breakers. Running multiple postgres with postgres-aware replication and failover is safer, faster, and more performant (though harder to set up).
