You can't possibly drag an SLR (let alone a telephoto lens) with you if you're gonna swing around like that.
On a more serious note, working upside down isn't that disorienting, but it's a bit uncomfortable when all your blood rushes to your head. No photographer should ever ask any model to do this for prolonged periods.
Also, note that hair and loose-fitting clothing will flip in the direction that gravity demands.
Photo taken as an entry for this month's GetYourGearOut photo competition (theme: self-portrait).
I wanted to submit this older shot from 2013, but the rules require that the picture be shot within the month of the competition. I've also picked up a bit of flash photography since then.