I'd wrap something around the bar that they backing goes on, so you have a bit of padding to pin into. Then I'd attach it to the top leader and float the entire quilt, holding the front edge by pinning it into the padded front rail.
At one point, I had "Loaded" about 10 yards of muslin on my backing rail, and found it was really nice having that extra padding, so instead of using it up, I started loading my backings on top of it. I float my tops, and having that to pin them to worked way better than using clips, though I did have to be careful to remove all the pins before rolling the quilt when I completed a section.