hello.
i needed yesterday this functionality for one of my clients who transferred from joomla to WP so i have quickly added filter in popup
you can get changed media-categories.php file from here:
http://pastebin.com/RM9iLTrw
basically i have removed default media library tab in popup and replaced it with custom one where i have added filter so there is no messing around with core code.
one thig is that in select list current category was not selected so i have change that also. now current category is selected
i have commented code so you can see what was changed. changes were made on following lines:
107-109
327-329
337
348-350
763-to the end
hope you will find it useful and that developer will clean it a bit for next release of this excellent plugin
