To answer my own question (for anyone who may need the answer):
Take a copy of jbosscmp-jdbc.xml which you can find in /server/default/conf as standardjbosscmp-jdbc.xml and place the file (renamed to jbosscmp-jdbc.xml) in your META-INF dir.
Modify the file according to your needs.
When you deploy the app it will use the jbosscmp-jdbc.xml in your META-INF.
You will still need to add realm information into your login-config.xml