Not really. Just looked into it. The plugin creates a new .htaccess file in the upload folder which redirects requests to all JPG images with the appropriate sizes (as set in the admin settings) to its own watermarked files (creating those on the spot if they do not exist already).
The rewrite rule for this uses a very long, complex regexp. You could theoretically fiddle with it and try to use special image file names. What will NOT work is using a special CSS class for exclusion, because filtering is done on the .htaccess level, not by PHP.
The easiest and most pragmatic solution in my case was to simply use a PNG file for images that should not be watermarked.
