Uhm, that doesnt even slightly resemble changing the music playing...
And as Stijn said, the boss event isnt made for that, and as I also said before, you'd have to include Boss1 and/or Boss2 in your pack, and so people will end up having like 20 of each, if not more. And that also only allows 2 of those events per pack, otherwise you'd have to change them in between before going to the next level, which will give the players extra confusion and make the pack worse overall.
Yes, there are custom boss music files on J2O, but only a couple. I actually liked the one included in Dreamscape by Blacky, and I replaced the old one by it totally since I like it better. But this only rarely is the case (as far as I know I've only ever downloaded 2 custom boss music files, one of which being the aforementioned, and I've downloaded a lot).
But enough about this, BlurredD is the boss, and only he decides to add it or not and how to do it.
Another idea I have is instead of referring to a text string, why not refer to a text file outside of JCS, I think that must be possible...