This sounds like an ideal use for MC's custom library fields. Make a library field like [Custom Playback Data] and put whatever text in it that you want. Then you can query that field using MCWS.
If you run the "Info" command in MCWS, it will give you a bunch of information about the currently playing file, including it's file key. Then you can use the file key with the "Get Info" command, and specify that file key. Be default it will return all library fields for that file. You can specify the field or fields that you want to see. They come back as XML, so they shouldn't be too hard to parse.
You could set up a process like this that polls MC every N seconds. Or perhaps you could link whatever you are using to choose movies to your polling program, causing it to poll MC shortly after the file is selected.
Brian.