It seems to me that there are, at a minimum, two transponders on 129 that could be dedicated to new national HD pretty easily right now:
Transponder 1: Had Kansas City HD Locals on it until last week, when they were moved to TP's 4 and 12, which are now 6 channels MPEG4 each. TP1 is now empty, and could host 6 national HD channels.
Transponder 11: Currently has 7 HD test channels on it, and it would seem like a good candidate for 6 HD nationals (or more, if they can do it).
Brad