From time to time, people on the OTN XMLDB forum, ask if it is possible to connect via the Oracle Protocol Server (aka the Oracle Listener), for instance in combination with APEX and DBMS_EPG. I am not sure if it is an already official supported environment, but the following works as demonstrated in this step-by-step manual
Oracle RDBMS 10.2.0.3.0 (Enterprise Edition) and/of higher version
It’s possible to have also two different version listeners (the 9 and 10 version) running, by settting different ORACLE_HOME’s and PATH’s, and starting the different listeners. I like to have 1 listener – as long as this can be maintained regarding incompatibility and bugs etc – therefore my latest listener version is leading (version 10g). Set your ORACLE_HOME accordingly. My 10g Oracle home is placed in directory/oracle/rdb10g/ and my TNS_ADMIN directory is set to directory /oracle/network/admin.