Even if it feels like a workaround for you,
your idea helped me out, thanks!
I don't know if it's the exact way you did it,
but after some figuring out
I added the following line to my theme's functions.php:
add_filter('the_content_rss', 'do_shortcode');
//processing shortcode for output in feeds
Maybe this will save someone some time scratching his head :)
