I'm trying to figure out how to export my project back from media composer 3 natively in mp4. Cilp browser will create the BPAV structure to save it and play it with the camera, if needed, the down conversion as well.

I don't believe Sony has enabled this like they have for XDCAM HD. As far as I know the only thing that can be written back to a SxS card is metadata using the Clip Browser software.

If I can create native xdcam ex mp4 files from avid, clip browser will do the rest. unfortunately avid doesn't do it. Using a main concept plug in for premiere for example, you can create mp4 files and the BPAV structure ready to be copied to SxS card and play it with the camera. A work around could be to export MXF OPAtom and than rewrap it to mp4, the other way aroun that clip browser does.