Generate lower case tag names with Oracle DBMS_XMLGEN?
According to ORACLE-BASE, 


  Where possible DBMS_XMLGEN should be used as it is more efficient than using the Java based DBMS_XMLQUERY


And:


  The example below is a copy of the previous code with all references to DBMS_XMLQUERY replaced by references to DBMS_XMLGEN. Note that two of the lines have been commented out as this functionality is currently not present in DBMS_XMLGEN


I added my emphasis, because this refers specifically to the settagcase you are looking for.


--DBMS_XMLGEN.settagcase(v_ctx, DBMS_XMLGen.LOWER_CASE);



So it looks like you are stuck with the slower DBMS_XMLQUERY.