First of all, not backstage, but:
Itte Yoshi Part One and Part Two. This is little improv bits that Hyoutei did, the setup being that Sakaki will say "____, you were slacking off in practice." And then the character will give some explanation or something, and then Sakaki will say "Itte Yoshi." XD;;; They're pretty funny.
How to play the Seigaku Game! We should try this sometime. XD;;; I don't quite get the rules, though...
Next, the backstages:
Original TeniMyu backstage! Watch for Kimeru in a headband. It's really sad to see Yanagi-baby dance and sing now, though...
More than Limit backstage Part One and Part Two! Watch for KENN stuffing his face, everyone stealing everyone's food, and Shiozawa's AMAZING "nfu~"!!!
Imperial Match Backastage Part 2 of 2. Sorry I couldn't find part one, but here's half the goodness! Watch out for Golden Pair, Silver Pair, and Pillar Pair (as in, two of my actor OTPs... XD;;) and Shirota and Yanagi-baby singing Katsu no wa Hyoutei together!!
KENN related Imperal Match backstage. If you ignore the subs in Norwegian, this is really enjoyable. Watch out of Ruito Aoyagi!KENN and KENN sobbing while Shirota does weird voices at him. XD;;;;
A compilation of a bunch of Hyoutei Backstage videos. This one is amazing, there's an ADORABLE Saitou/Aoyagi moment and a cute KenKen/Date one, too~
Aaaaand that's all, folks! I know, it's the best thing since NYA NYA NYA NYA! Lemme know if you have any other links and I'll slap 'em up here! [This is only for the musicals I've seen, btw. I'll watch Rikkai and things when I see RikkaiMyu.]