I had a very similar issue not so long ago, albeit, my scenario was allowing the user to select images by type of room, color, feature etc..
Using inspiration form this site: Inspiration and a combination of these two plugins, Custom Upload Dir, Media Tags it worked out quite well.
I dont think your going to be able to do what you want though without going through every image and tagging it in some form or other.
Again I had the same issue as developing a site AFTER uploading images. In the end I categorized all the images by type into folders on pc, then selected all images and whilst in windows,adding a tag to the copyright exif.. couldn’t workout how to get the keyword exif 🙂
Then using this piece of code, the copyright exif info got written to the meta tag (tag) on upload.
add_filter('wp_read_image_metadata', 'hk_filter_add_exif','',3);
//******************************************************************************************
// this hook function adds also the exif tag "copyright" in the database of each uploaded image
//******************************************************************************************
function hk_filter_add_exif($meta, $file, $sourceImageType)
{
if ( is_callable('exif_read_data') &&
in_array($sourceImageType, apply_filters('wp_read_image_metadata_types', array(IMAGETYPE_JPEG, IMAGETYPE_TIFF_II, IMAGETYPE_TIFF_MM)) ) )
{
$exif = @exif_read_data( $file );
if (!empty($exif['Copyright'])) $meta['tags'] = $exif['Copyright'] ;
return $meta;
}
}
Could then use standard wordpress functions to get this tag info to use in the plugins.
This isnt an answer as such, but hopefully it may give you ideas.