WOW! this really rocks!
the things i tried seemed to work fine.
i noticed the following:
you seem to allow to set the brandname only on video streams. this is problematic as with audio-only mp4 files the user cant set a brand and with mp4 files with more than one video stream he could set more than one.
if possible it might be better to allow setting the brand on the first stream to be added no matter what type it is?
also it might be good to give the user the option to set a brand called something like "MPEG-4 compatible", meaning letting mp4box set the right brand it wants to use (also pointing out that using an own brand is not really mpeg-4 compatible)