One way to deal with a thing like that, I have myself be doing a long time.
I serve my css file like this:
<link rel="stylesheet" type="text/css" href="css.php?ver=2">
Then, within the server side code, I do several replacements based on what version passed as argument, some browser sniffing whether to embed data in url's, like url(data:image/png;base64,...) and so on.
For your needs, you could mark your css values like this
background:url('/cdn_img/sprite_social.png')
and then it will be easy to do the replacement and change the "/cdn_" to "cdn.domain.com" or whatever you need.
I also use this solution to merge many css together into one, where I save a lot of callbacks to the server, one otherwise get with many link tags.
P.S. Using this, don't forget to set the ContentType
to text/css