Is there any way to apply open-ended keyword tags to songs in iTunes? Other iLife apps like iPhoto already support this functionality, like assigning "vacation" or "mountains" keywords to certain photos.

Is it possible to add tags to songs in iTunes without resorting to the rigid options currently provided (i.e. the 'Comments' or 'Grouping' fields)? See this mock-up for an idea of what this type of tagging would be like:



In the above mock-up, we could assign sub-genres to a song. This would allow us to create smart playlists based on these sub-genres or do whatever else we'd like with them.

Currently, someone would have to use comma-separated values in the 'Comments' or 'Groupings' field to apply this type of sub-genre tagging. It's very inflexible and annoying.

Why doesn't this type of tagging exist? Can it be implemented with plugins?
It currently is not possible to add tags in iTunes like you can in other iLife applications. The only reason I can think of why its not available is that most people would just group their music by genre, and that be enough. 

Other than using the comments field or the like, there are really not many other options. In some searches I did come across an app called Qloud, but it seems to no longer exist.