Well, if I can't get help, I tend to solve things myself. ;)
For anyone interested, here's how to hide namespaces in the Media Manager for which the user does not have at least Read permissions:
In the file
inc/media.php: in
function media_nstree($ns){, add this before the echo statements:
// Remove namespaces for which user does not have at least read perms.
foreach($data as $key => $namespace){
if(auth_quickaclcheck($namespace['id'].":*") < 1){
unset($data[$key]);
}
}