The Oracle cartridge interface allows for domain index operators to do full index searches or row level searches. The cartridge must implement both. In this cartridge, both implementations do a full table search using the RMI server (even for the row level search a few hundred RMI call-outs are going to take longer than a complete search).
It appears that the optimizer always chooses a full index scan (though this could change in the future). Dbms_output messages indicate which implementation is used when a domain operator is used.