I was running into the cache issue and I don't really want to stop the css file from being cached so I ended up going with the querystring option which works like a charm.
I was just trying to see if I could find a way to do it with all of the dynamic code within the css file itself. I think what I learned is that the <link> tag makes that impossible, but it does make it easier for the browser to know that using the cached version is ok.
So it was a fun experiment and I learned something but in the end I don't think it's possible to detect where the call is coming from within the css file itself.
Thanks for all of the input!