well, yes, that certainly does work, but it's a pain...
I have figured out how to at least get the htaccess rules to work so the dynamic css has a fancy url. Here's the example I used for the jquery mega menu plugin - this htaccess file goes on the same level as the skin.php file:
RewriteRule ^([^/]+)-([^/]+)\.css$ skin.php?widget_id=$1&skin=$2 [NC]
Now, the trick is to replace the reference to that file to use the fancy url.
There's probably a way to do it without hacking up the plugin, but since this particular plugin isn't using any hooks to enqueue the css, I'm just going to modify the plugin to make it use the fancy url.
It's currently working and the file got picked up by the cdn! woohoo!
I wonder how hard it would be to package something like this into a separate plugin to "fix" dynamic css files like this.