First of all, I don't know whether or not this is possible, but I'd really like to see such a feature.

Let's begin with an use case: I often listen to my music in random mode. There are songs I don't like as much as other and so I often end up right clicking on the notification area and selecting next, sometimes several times till I find a song I like. (Same use case with a given album is also true: you rarely like the whole album, there are often a few songs you like less)

What I'd like is to be able to pass the current song directly from notification. The way I see that would be to have a small button "next song" (like the one in the toolbar) in the libnotify box, so that it would allow to quickly jump to the next song, without requiring so many clicks.

Here is how I imagine it :

+----------------------------/\------+
| Now Playing                      X |
|   A little less conversation by    |
|   Elvis Presley                    |
|                              << >> |
+--++--------------------------------+

Where << allows one to replay the last finished song and >> permits to go to the next one without playing the current one at all.