Shaun:
Did you ever get any satisfactory resolution on this? We're dealing with the same issue, where we can't find a place to modify the URL that's returned in JSON output from ArcGIS Server map export requests.
I have discovered, entirely accidentally, that ArcGIS Server 10.1 can automatically change that URL for you, to match the URL that you used to make the original export request.
We have a couple of reverse proxy servers and firewalls in front of our ArcGIS Server (AGS) box, but enough of the original export URL seems to be getting to AGS for it to change the URL in the JSON output to match. I'll speculate that the info is coming in on the HTTP headers, so your reverse proxy(s) probably needs to preserve those.
In our case the URL in the JSON is a bit mangled right now, but is very close. Working to fix that now.
I sure wish this behavior was documented somewhere. Anyone?
David