Nope, PIE doesn't support background-size yet.
You might be able to use border-image with the 'fill' keyword, if you give it slice widths of 0... that should make the center piece expand to the size of the target element. Haven't tried this though, let me know if it works or not.
The only other thing I can think of doesn't involve PIE: give your container element position:relative, and add an <img> element inside it with styles like:
Code:
position:absolute;
top:0;
left:0;
width:100%;
height:100%;
z-index:-1;