I'm assessing the practicalities of developing an app that uses private APIs for personal use and potentially release in Cydia. Specifically I'd like to understand how easy it is to add music to the Music library. Half an hour of googling has revealed this, but I'm not convinced I'll find what I need without lots of trial and error. Are there any hints out there as to how to get going with this task... I can't help thinking a number of people must have tackled this before.
The best insight I was able to find was this open source library by a fellow Stackoverflower:
https://github.com/H2CO3/libipodimport
It uses some of the private APIs in the link you found, but also seems to create a hook into the Springboard--perhaps only the Springboard app can access the music library (or it could be just because the author wanted it to be a persisent process).
You could contact the author/s and find out where he/they got the information, or check the README for details on the sources (twitter handles).