Is there a PostgreSQL proxy running queries on several servers at once? Let's assume we had PostgreSQL version X.Y and decided to give a test-try to version Z.W. Probably, there are some changes between X.Y and Z.W that could cause a little bit of trouble :-)

It would be preferable to run Z.W in parallel with X.Y, checking whether it goes well and using only X.Y's replies for a while. Later on, having Z.W's DB "ok" we could use it as primary, discarding X.Y optionally.

Question: Is there any PostgreSQL proxy that can run queries on several servers at once?
Have you checked pgpool already? It has query replication as a feature (look for 'parallel query').