I would see it that way:
We add custom music (wav or ogg when implemented) to specific folder, it is detected on startup and a certain file file is played in-game as a background music (like, say, Crimsonland does) and Now playing (filename) appears in the console. There could be also options for playing tracks random or in specific order (alphabetic in folder presumably). Maybe even a playlist like in Tony Hawk's series, where you could temporarily disable a certain track. IIRC, ogg does have file info similar to idv3 tags in mp3 to extract.
I'm not sure about interference with existing music zones in certain maps, but I think music could be temporarily muted and paused, plus these are few and far between I think
I agree that there are more important things to implement (better in-game server browser for one), but this would be nice to see imo.
Regards.