If you want to fetch a subtree no matter the permissions, you need add a parameter array(Limitation => array()) to the subtree function. Have a look at the code ofeZContentObjectTreeNode::subtree for clarification.
If you are not adverse to a bit of kernel hacking, you could try to change the module.php in kernel/content. Change the value of functions in the variable $ViewList['download'] to e.g 'my_download'. In the functionList below copy the entry for $FunctionList['read'] and change the key to 'my_download'. Now you can set the permissions for access to the download view the way you want. Please note that I have never tried this for the download, so test this first and keep a backup of the original module.php!
