That would work, but there is a chance that by the time you have quilted all of it, the edges will probably have shifted some, and I seriously doubt that all the edges will line up exactly, so all four sides may need to be trimmed a little. If that's not a problem with your design, you could certainly give it a try.
I'll show you what I mean. I recently did this quilt on my long-arm. This is the pieced back and I was able to get the center of the back lined up absolutely with the center of the front. It was loaded so that I started quilting at the bottom edge of the quilt, and by the time I got to the top, the backing had stretched a little in the center, and you can see that the seam at the top is not exactly straight, but has a slight bow in it. If I had been more aware, I might have been able to ease in that 1/2", but it may have caused tucks in the backing, and I think that this is less obvious. If the top and backing had been the same size, this side would have needed trimming. As it is, this is the back, and I said that "finished is better than perfect!"