I just created a playlist on a library server client. Quit MC and the playlist appears on the server, and re-appears on the client. I added a track to the playlist, and quit MC, and the playlist is correct upon restart of the client MC. The server's version is correct too.
Same thing with a smartlist.
The key is to set Tools > Options > Media Network > Automatically sync changes made on the client back to the server
Syncing is hinted at when you look at Library Manager's UI (File > Library, and select your library server's name). You see a button for Sync Changes with Library Server.
Note: there is an button to look at "Library Options", but this explores the currently selected libraries options. There is no button to configure the connected client's options. I think there should also be a LS Client Options button, which is a shortcut means to open and discover Options > Media Network > Client Options (when connected to a Library Server) and would show all the client options. This would allow users to see "Automatically sync...", etc. client options.