There is need to put SQL comment before the SELECT statement with Pgpool-II config:
If you don’t want a query that qualifies for the load balancing to be load balanced by Pgpool-II, you can put arbitrary comment( /(asterisk)NO LOAD BALANCE(asterisk)/ is usually used) before the SELECT statement. This will disable the load balance of the particular query and Pgpool-II will send it to the main node (the primary node in Native Replication mode).
With annotate method is only to possible put SQL comment after the query.
If idea is acceptable I would be happy to start coding this feature.