Caching Externally Linked Images

I’ve done some googling and I can’t seem to find the magic method for setting this up. What I’ve got:

1) Photos hosted at an external hosting service (SmugMug).
2) Posts with img tags that reference the externally hosted images.
3) A Rackspace Cloud account.
4) W3 working great for everything but the external photos. Everything uploaded to Rackspace like a champ (awesome job, by the way).

What I’d like to have happen (why I am here):
The ability to upload the externally linked images from SmugMug in to RackSpace Cloud while W3 handles all the URL juggling, etc. Does that make sense?

If someone can please point me in the right direction I’d be appreciative. Maybe it just isn’t possible and I’ve misunderstood what this thing can do. In case it should work, here is what I have tried:

Under CDN config options:
1) Checking “Import external media library attachments”
2) Checking “Export changed files automatically”
3) Adding *.jpg to “File types to import”
4) Checking “Host custom files”
5) Saving and trying to upload multiple times.

http://wordpress.org/extend/plugins/w3-total-cache/