I would assume that if the drivers in the MB Twin exposed a method to do AAC to DD transcode, then it would be included in the image menu as it is in the Quad Plus. The facility is there for other boxes so no additional development needed I would have thought. Someone with in-depth understanding of the underlying image software may comment perhaps?

You could always try another image to see if AAC to DD transcode is offered. It's just a flash job. You could make a backup of your ViX image first (ViX option in System menu) so that you can easily revert. I had tried other images for the PiP restriction (SD only in PiP) as I had thought that it should support HD in the picture window according to how I had read the specs in the manual, but no image supported HD PiP.