I don't think having a client sided add on would do any better.
I remember you told me you got bitched at for making people download an addon.
Yeah, and it'd be a terriable idea to make people get an add-on during the actual event.
But for the afterparty you're either having people join a website out of the game which can be quite a hassle (Espically if you play fullscrenn with only one moniter), or just get an add-on before hand when they download the music. Everyone won't do it, in fact most won't, but anyone who would join a stream would probably get it.
Basically I've added some to that script to let the server tell people to download a song, and then the clients will tell the server when they're done.
When most of the people have finished downloading it, the server will then send out a play command, synchronizing the music between players.