MahoppianGoon released x360 Music Organizer v0.01 beta:

Here is a small program I've been working on only since July 4, 2010.
It was originally going to be a music organizer for custom music on your Xbox 360, but I soon realized how difficult that 'mindex.xmi' is to read, so I gave up on that for now.
Right now it just creates a NEW 'mindex.xmi' file and the 'FMIM' music files. (maybe one day it will be a full music organizer, don't know just yet)

Description:
+ Create a custom 'mindex.xmi' file and 'FMIM' music files!
+ Automatically creates the 'mindex' and 'media' folders!
+ Creates a NEW 'mindex' folder and 'mindex.xmi' file, so backup your current 'mindex' folder on your Xbox 360 first!
+ Tested with 50 songs, and all worked perfectly. So you should be able to add as many songs as you have space (limit might be 25,000)
+ Even though you can only add 1 artist/album right now, you could use the 'Filename' as the 'Song Title' and then make Playlists.
Official Site: n/a, by MahoppianGoon
Download: here