I have a WAR file containing an XML file and I have a singleton class that, on first access, opens the xml file and configures itself from it.
org.xml.sax.InputSource is = new InputSource("http://localhost:8080/mywar/myfile.xml");
This is a classic example of a sledegehammer for a simple nail. There are plenty of ways to do this without a resource adapter.
I would suggest asking one of the WebBased forums for advice on something like this.
Also, a custom MBean might be a good choice as well.