Postgres "session variables" kludge on AWS RDS
AWS Support confirmed that it is, indeed, impossible to set defaults for custom session variables on RDS PostgreSQL.

The next best solution was to trap SQLSTATE 42704 (undefined_object) errors in all the trigger functions with


  EXCEPTION WHEN undefined_object THEN SET kludge.param TO 'default_value';


followed by replicated trigger code, so that the trigger gets properly executed even in the error case.