Oracle Designer 10G now seems to have an extension that will allow you to generate the source code of your choice.
Extend Designer generation capabilities using Oracle Designer Extension Builder (ODEB). Generate the source code of your choice from the Designer metadata based on user-defined templates. ODEB also provides a way to export solutions (Driver, associated Templates, Parameters, Tokens) which can be then imported back to a ODEB schema providing a way for people to share their solutions with others in the ODEB user community.
Will this be a competitor for CodeGen? Maybe, if you’re using Designer anyway then it could be. If you’re not using Designer (yet) then I think CodeGen is your best shot at generating code.