I’ve seen the use of SelectedXYZ in MadMapper and also in Resolume, with the Namespace repeated of the selected item (Though Resolume doesn’t support OSC Query).
Selection can appear in two contexts I can think of:
Having e.g. a static TouchOSC/Lemur GUI, for SelectedXYZ, along with an Increment/Decrement selection button, so that all XYZ items can be controlled from the same GUI.
Selecting one or more items, for a sub-sequent move operation.
Actually, this doesn’t need to be communicated – it can be handled by the OSC Client ONLY, and the operations be serialized to the Server.
If only one XYZ CLASS Instance is ever selected, I guess an ATTRIBUTE / TAG, that it is a Selection duplicate of some other item in the CLASS hierarchy is sufficient!