How to modify an OBB file generated with Unity 4 ?

Hello,

I am trying to modify the OBB file that is produced with Unity 4 when the Split Application Binary option is checked. So far, I understand that it is not a simple OBB file created with the jobb tool from Android SDK but actually a more complex process done by Unity. It appears to be a zip compression but somehow random.

From two builds done with Unity, one after the other and without any single modification, scenes files (level0, level1, ...) will have the same uncompressed size, but most of the time, different compressed size (about a few bits). And they are the only files that are subject to this size difference. It might not be relevant but still, I don't understand it.

What I understand though is that there is a 32 characters long hexadecimal tag in the assets/bin/Data/settings.xml file in the APK that represents the MD5 of the OBB. But even though I am getting the new MD5 once the OBB modified, changing it in the XML file, resigning and optimizing the APK (I know I got that part working since I can already run a modified APK and read added files), I am still getting this "Level 'LevelName' (1) couldn't be loaded because it has not been added to the build settings" error.

There must be some signing, optimizing or packaging process that must be done with the OBB file that I don't get. So I am asking for your help, hoping someone knows :)