Sure wish I could hear what song they are messing around with. Looks like Keith is in standard tuning; Taylor seems to join in on roughly same chords. I cant make out what Jagger is saying/ singing. I love the backstage clips from 68-71.
Those are in fact the chords to the chorus of Wild Horses that Kieth is playing: Am - C - D - G - F - C. It's very clear when they get back together following Mick and Mick talking.