Bike porn

Not that one I know, but they seem to have done a couple with carbon seat tubes. I guess the logic is weight + a bit of comfort in a part of the frame that doesn't take as much pain?

Is it common in other custom MTB frames?

I havent seen it much. I would say it was done firstly to look cool (which it does), and secondly to save weight.

MrStrong has done something similar (yes this is a repost, I posted it previously)
YouTube - NAHBS - Strong Frames