I know this is putting the cart before the horse, but, if the Cowboys DO sign Asomugha at CB why not try Newman at FS? He's not a killer at tackling, but he's not too bad either, at least in the open field. Playing S and CB are 2 entirely different things I know, but he's smart enough and savvy enough to make the jump.
You keep his experience on the team, he can slide to the slot if needed (he did that last year) and provide some backup at CB in an emergency. I don't know if he'd like it or not, but it would be better than being released. That would leave Jenkins, Asomugha, Ball and someone else at CB. Of course if Asomugha signs somewhere else it's a moot point.