Once the feature classes get created correctly in Oracle, why not extract the DDL for the indexes, drop them, and re-create them after modifying them with the desired names? It's not an ideal workflow, perhaps due to the number of indexes involved but you could do it technically. This can be done via SQL Plus if you don't have a nicer tool like DBArtisan or TOAD:
SELECT dbms_metadata.get_ddl('INDEX', 'IDX_NAME', 'SCHEMA') FROM dual;
DBMS_METADATA.GET_DDL('INDEX','IDX_NAME','SCHEMA');
What is the reason why you absolutely have to retain the exact same index names from your Diagrammer XML file when importing it into Oracle?