I don't know much about how OpenLaszlo works, but presumably you're wanting to use SilverStripe to generate its XML configuration files?
There would be 2 ways of doing this:
1) Make your own templates that return XML instead of HTML. You could make them the default templates, or you could call the templates something Like Page_xml.ss or MyType_xml.ss, in which case, the XML format for the page would be accessible as http://www.mysite.com/(pagename)/xml
2) Make a subclass of controller called OpenLaszlo, and then different methods can be called on this by visiting http://www.mysite.com/OpenLaszlo/(method-name)