There's a network share functionality within USB tools of RUT955, but I'm not sure if that's going to be helpful with DLNA clients.
There's a package called minidlna in opkg list, you can download it and install it, I believe it should work normally with USB storage key, you will just have to change directories in config.
More about it you can find here: https://help.ubuntu.com/community/MiniDLNA
Make sure to do opkg update, before looking for minidlna in opkg.
Also, keep in mind that this is a third party package and we're not responsible for it.