This may sound naive, but implementing your own pooler is very easy e.g. in Go pgproto3 already does all packet-parsing for you. Transaction poolers are looking on ReadyForQuery packet and it's "in trnsaction" property like this [0]. All you need - is stick server connection on new ParameterStatus[1] packet for "SET search_path" instead of ReadyForQuery.

[0] https://github.com/pg-sharding/spqr/blob/358f816cd8a964a9c9e... [1] https://www.postgresql.org/docs/10/protocol-flow.html#id-1.1...