You could use ActionScript to load the XML from the other sites.
When getting data via ActionScript you can use the async URLLoader or use a JavaScript function via the synchronous ExternalInterface.call. You can find information about both methods at www.actionscript.org.
URLLoader Example
var loader:URLLoader = new URLLoader();
loader.addEventListener( IOErrorEvent.IO_ERROR, this.ioError );
loader.addEventListener( Event.COMPLETE, xmlLoaded );
var request:URLRequest = new URLRequest(file);
loader.load(request);
ExternalInterface Example
if (ExternalInterface.available)
return ExternalInterface.call(jsFunctionName, optionalArgs);