One other thought to add ... if the users are in one location and have somekind of connection to the internet (as well as a local TCP/IP network), wonder if rsyncing the 'right pieces' of the hosted moodle site to a local web server in the wee-hours of the AM (when what bandwidth is available is not being used by users) would do.
There would be some edits/changes one would need to make with config.php for the remote moodle servers. The rsync initial run would take a long time, but with rsync set up to do incremental, only those items changed are xfer'd the next rsync run.
Could imagine this to work best with Moodle 1.9 as the course ID directories would contain the multimedia files. IF using Moodle 2, the multimedia files would have to be in folders outside of Moodle to prevent them from being included in the DB.
Also, and I don't know this is possible with Moodle code, relative addressing of multimedia resources in Moodle would be best rather than full URL to resources. Example: at the document root of the web server a multimedia folder with subfolders of audio and video. A relative link would look like: /mutimedia/audio/somemp3audio.mp3 Apache would look in document root /multimedia/audio/ for the file.
Just thinking outloud ... never tried it myself. ;)
'spirit of sharing', Ken