Yes its a standard tag. Its included among the tags that are available for audio files. I think I'm working like you. I'm taging on a client machine and the mp3 files resides on a server.
I did some more test and it seems like there is a problem if I use the Country tag before I import it to the library (in Playing Now). I use the Library Tools to move the mp3 file to a specific place on the server and perhaps this mess things up when I start taging before its moved. If I tag the file after its moved it seems to work. However I rather do all the taging before I move the file.
But as I said before, other tags like Artists, Album works fine. Strange problem.