Actually what we have done in our instance is to define a parameter in our config.tcl under the ns/parameters section named: ReverseProxyMode (boolean).
Then on util_current_location proc, in case the parameter is set to true, two HTTP header fields are checked : X-Forwarded-For and X-SSL-Request. Depending on this checking the returned protocol is https or something else ( http usually ).
I quickly generated a patch against HEAD, you can try it out:
https://gist.github.com/6ae69309b3ece8c15603
