If your going to weigh a snake I think a bag is much more simple than your suggestion. Your suggestion is good until the snake decides it is leaving. My gtp is pretty good and will usually lay in the bowl without complaint. I don't think that being bagged is going to stress it any more than removing it on the perch and carrying it around. A bag is like a hide to a snake. They will usually crawl in by themselves.
I have a feeling I'm about to be very jealous!