Works like a charm.
However, when using the server-side method, non-image uploads (like PDF documents) are broken.
I had to edit the regex in the rewrite rule as follows:
RewriteRule ^files/(.+.(?:jpe?g|gif|png)) wp-content/plugins/wp-retina-2x/wr2x_image.php?ms=true&file=$1 [L]
Now it only applies to image files.
Would be great if this was included in a future update!