As you stated it, the only way is to use a config.xml for each of the two roles. Unfortunately the current version of the BigBlueButtonBN plugin does not support the use of BBB dynamic config.xml.
You would need to implement it making the changes to the Moodle plugin.
There are some examples you can use, but they are in Java
http://demo.bigbluebutton.org/demo/demo12.jsp
And the documentation for this API feature is here https://code.google.com/p/bigbluebutton/wiki/API