I have to admit, I'm impressed that the character mods that are going beyond simple texture swapping (visually, I mean, I know move hacking has been done for ages). In particular, there are a few very convincing Rosalina hacks, such as this
As of today, I've successfully Multi-Man Brawled to the SSB64 Metal Mario music. Works like a charm.
Of course, any files to be replaced need to be in the format that the game uses. For Brawl music, that's .brstm. You can find a friendly converter for making your own right here:http://blog.dantarion.com/brawl-custom-song-maker-gui/
Two things to bear in mind with this thing:
1. Looping is supported, but your loop point is created by manually entering the sample number, which you will have to figure out for yourself.
2. It's anal about using a file structure for older Brawl hacks on your SD card, so you'll need to send them to the correct folder manually (although it does automatically name your file for the music track you want to replace, which is cool). The correct folder for replacing Brawl music in Riivolution, from the root of your SD card, is ssbb/sound/strm/
So that just leaves Riivolution's XML to tackle. Fortunately, their guys have a page with some basic templates:http://rvlution.net/riiv/Patch_Templates
The examples posted there will auto-load any replacement files they find for their respective game, and Brawl is among them (the other three they have so far are Mario Kart Wii, New Super Mario Bros. Wii, and Tatsunoko Vs. Capcom, and from what I can see changing them looks very simple). Download the XML you want, make a "riivolution" folder in the root of your SD card, and put it in there. You're good to go.