I finally got around to watching the recording I made from the Ku-band distribution feed (7.5Mbps MPEG4), and the only time I saw banding and artifacting was during some but not all close-up shots of the fireworks. It seemed to happen only when there was nothing but fireworks, no background. All the other scenes were gorgeous PQ. (But yeah, the bad parts were BAD.)
Since I only saw a problem in some shots, I wonder if the MPEG4 encoder was having a problem with dark scenes (they only just switched to MPEG4 within the past month!), or whether the problem was with one specific camera.