Interesting. So you guys are close to adding your own songs as DLC? Cool stuff guys. Maybe I can help out. I have a bit of... experience with this kind of stuff (adding DLC to games). I know it's a bit different for each game, but I know the basics (such as building contentmeta.xbx and contentimage.xbx files and getting them signed).
I've worked with the Mimesis team before about creating support a few games (KotOR, Black Arrow, and soon GR2 and PGR2) and have had good success. I know the Mimesis team is always looking to expand the number of games supported by Mimesis and I think that this game would be no exception. If anyone wants, I could take a look at the DLC for this game and try to figure something out.
By the way, I've read that the data is already on the DVD and the DLC just unlocks it. This is not entirely true. The package that goes into "$C" might unlock the package, but it doesn't mean that the data is on the DVD. There is one game I know that is a perfect example of this (not going to get into specifics). The data (along with a few updated files that add it, ussually a .ini file (it was listed in here somewhere, but I don't remember it's name)) can sometimes go into "$U". Anyway, if anyone wants me to help out, let me know.