Unfortunately, I don't believe there's any API in mala that will notify a plugin of the currently playing song.
There's a ScreenSaverSwitch callback, which is called as the screensaver switches to a new file, but that's not necessarily background music notifications.
HOWEVER, it looks like if you read the MALA.INI file, there is a LASTSONG=entry with a number, add 1 to that number, and then look in the MALA.PLS file, for FILE##, and that should tell you the currently playing (or lasted played if mala isn't running) song.
The downside to that approach is you have to poll the INI file constantly to detect when it changes.