> Is this possible?

yes. depending on your setup it could worth the
try to use nginx as static server for your download-files,
esp. if you run your proxy_pass - location on on the
same server. os-cache can be as fast as a ram-cache
via tmpfs.


depending on the amount of files in /download and filesize
it can be useful to tweak your setup (buffers, sendfile etc)



http://wiki.nginx.org/HttpProxyModule#proxy_cache