I think that for PHP-less downloads to work the administrator would need to be able to enable the feature per-forum, with the caveat that it means all users will be able to download the files. This way we can enable it for public forums, but leave the protection enabled in private forums.
Alternatively, it could automatically be enabled if all users have permission to download files from the forum; i.e - if guests, and all listed user-groups have that permission enabled along with the ability to read the contents of the forum. Ideally, if the forum's permissions change such that the files are no longer "public", then they would be relocated to a private folder so direct links will stop working; I expect moving the files should be more widely supported than symbolic links?
Support for HTTP range in the PHP script might help, but I still find that PHP file masking scripts are rarely cached properly so HTTP direct downloads are still desirable for serving up content that is accessed commonly.

