Here is one possible way of doing this.
Place the properties (or data) file into server/xxx/data. (I placed mine in a subdirectory such as server/xxx/data/mystuff)
In your program, use the jboss.server.data.dir property to get the path for the data directory, and use that to access the properties file. For example:
String dataDir = System.getProperty("jboss.server.data.dir");
Properties prop = Properties.load(new FileInputStream(dataDir + "/mystuff/my.properties"));