I just auditioned the 5/6 channels on the 7.1 mix and found this. Nothing spectacular by any means, but interesting none the less.
I am thinking that taking the F-R-C-RS-LS of the 7.1 (tossing 5/6) might make the best 5.1 of the lot!
MOTHER: Piano w/Vocal Bleed
HOLD ON: Ambience
I FOUND OUT: Vocal
WORKING CLASS HERO: Ambience
ISOLATION: Bass Drum, Organ, Piano, Ambient Vocals
REMEMBER: Echo Piano, Out of time Vocals
LOVE: Echo Piano, Ambient Vocals
WELL WELL WELL: Bass Drum, Percussion, Vocals
LOOK AT ME: Ambience
GOD: Piano
GIVE PEACE A CHANCE: Ambience
COLD TURKEY: Silent except for a few sections of guitar only (Look at the wav above and you can "see" the guitar)
INSTANT KARMA: Ambience, but almost full background vocals on the chorus
With the 7.1 TrueHD stream of Gimme Some Truth, I use Audacity to combine channel LS with LR and channel RS with RR. The resultant mix sounded very close if not identical to the fold down of the Atmos to 5.1 from the disc. I might try both your method and mine with POB and see which I prefer. (Of course, I would actually have to buy the set first. )