Hi Pedro-- In UMA1 (RSR Sec 2.1) the resource description had a uri parameter that could putatively be used to populate discovery service entries, either directly in a Webfinger resource or through indirection. We removed it in UMA2 due to potential security and privacy considerations, but labeled it an area where people may want to add an extension.
Of course, such a mechanism would be useful if the AS were used as a central discovery service for the RO. The RS itself could make the RO's resources discoverable any other way it wants to.