Added two more things:
No live buffer captured on 922 when watching a recorded show
Scoreguide on my HR21 was nice too
Technically it does buffer... but only on one tuner. All of the Dish DVRs have this particular quirk.
Put tuner 1 on Syfy and swap to the other tuner and put tuner 2 on USA.
Swap back to tuner 1 and then go into the DVR menu and watch a recorded movie.
After the movie, you are correct that Syfy will not have buffered and you can't rewind from before your playback ended... BUT if you swap to tuner 2 you will find you can go back in the buffer there.
I use this "trick" all the time if I want to watch something on DVR before watching a show that is coming on soon... I'll put the "other" tuner on that channel and then swap back before watching my DVR event.