Phonon: subclassing QIODevice

Hey Forum,
I'm googling for almost one week now and I wonder wether its possible to subclass QIODevice to get Phonon::MediaSource( QIODevice* ) working.
I have a file where the header of the video starts at offset x. The docs say that I have to subclass QIODevice, however, I havent managed to get my subclass working: The virtual read/write methods aren't even called, why is that? Do I have to catch/emit signals in my subclass?
Please don't tell me to simply buffer the whole file, that hardly works with ~700MB.

Has anybody successfully tried to subclass QIODevice to use with Phonon?

Help appreciated,
MuFFin