the playingart link quoted (is covered in the docs with the current DNAS) is the way to get it outside of the stream data. but like you note, it then becomes specific to SC 2.x as well as relying on the source providing it to the stream to begin with (so not at all that different even if the DNAS was able to accept PUT requests, it would still depend on the source supporting such a mode, be it direct or via an auto-dj or via the native SC2.x protocol). is all somewhat off a mess and without any notable client support, it's just viewed low priority on the SC side to do much else with it.