I have absolutely no inside knowledge here whatsoever, just pure speculation, but I would not be surprised if BTN HD programming is indeed available in MPEG-2 on the HR10-250.
I highly doubt the BTN is going to broadcast full-time in HD. If anything, they will only offer HD games in three-hour blocks on Saturdays, in which case it would not be surprsing to see DirecTV throw those up on one of the HD spillover channels (94 or 95) that they use for things like NFL Network games and the occasional MLB, NBA or NHL broadcast.