For a variety of reasons this is incredibly difficult. Functions, etc make SELECT queries writes, not just UPDATE/DELETE, etc.
It's a lot easier for your application to know what a write is and just establish connections to 2 separate poolers (or hosts on the same poolers) and direct the reads/writes appropriately.
There's already working part of libpq protocol for this - target_session_attrs. But the problem with target_session_attrs is that it just takes too long to discover new primary after failolver. We want to fix this within Odyssey.
It's a lot easier for your application to know what a write is and just establish connections to 2 separate poolers (or hosts on the same poolers) and direct the reads/writes appropriately.