I can not try to do what you suggested cause I haven't got the plugin installed. I was thinking the same thing as you, to place a htaccess in the uploads directory (or wp-content) but when I tryed that earlyer it broke my theme as my timthumb.php needs to access the images directly to create thumbnails. So that won't work for me unfortunately.
I was hoping to find some plugin or solution to let you share selected data and files only with persons you send a link (or make member) and NOT with anyone else. The files must therefore NOT be accessible from a direct link without permissions.
You got any other solution?