I was told that Zimbra doesn't support modifying nginx to support this and that it would probably be simpler and easier to setup a manual redirect to a mail store. Here is how I quickly achieved this on my Ubuntu proxy node
apt-get install lighttpd
edit your /etc/lighttpd/lighttpd.conf file. Here's a diff between the stock and my modified one. This conf will take ANY request on port 7071 and redirect to https://zimbra.mailstore.com:7071/zimbraAdmin/
Lastly you will want to install your cert. I have a commercial cert on my proxy so I made a copy like this
> # "mod_redirect",
< server.port = 7071
> # server.port = 81
< url.redirect = ( "^.*$" => "https://zimbra.mailstore.com:7071/zimbraAdmin/" )
< ssl.engine = "enable"
< ssl.pemfile = "/etc/lighttpd/certs/lighttpd.pem"
cat /opt/zimbra/ssl/zimbra/commercial/commercial.key >/etc/conf/lighttpd/certs/lighttpd.pem
cat /opt/zimbra/ssl/zimbra/commercial/commercial.crt >>/etc/conf/lighttpd/certs/lighttpd.pem
/etc/init.d/lighttpd restart and you should be in business.