It’s far from perfect. How blurs are executed by CSS filters solicited some tricks to make them completely fill the background, and a trained eye will see some bleed on the edges. If you were to try different border sizes, etc. You would likely want to fudge with the numbers a bit to get appropriate coverage for the blur effect.
Let me know if you use this somewhere, I’d love to see it in action.
Update: It looks like you’ll be able to accomplish this effect with no tricks before too long.