I would like to know if it is possible to manually upload images to WordPress (ftp/scp) and make it recognize them and add them to the media browser. This also has to work with Wordpress running in multi-site mode (network).
I haven't tested this in multi-site but can't see any reason that it wouldn't work. The Add from Server plugin searches for images that you manually upload to WordPress and adds them to the media manager. You can find it here: http://wordpress.org/extend/plugins/add-from-server/ Tested on the latest WordPress.