Here's one possibility. Have the web service open a properties file and get the info about the file to load from there. You can either place the properties file in the web service archive, in which case you would load it as any other resource (and you would probably want to deploy using an exploded archive), or you could place the properties file in the server/xxx/conf directory and use the jboss.server.config.url property to access it.
I don't want to put a property file in the archive (not accessible enough), but a file in the conf directory would work. I thought of something like that but was looking at the data directory. I suppose the the conf directory is the 'official' place to put such things?
Either conf or data would work.