If you cant open the firewall, you most likely have to look at how to either encapsulate traffic in HTTP packets ( I see more and more layer 7 devices that will block this however), or modify them to accept HTML GET / PUT methods, or query strings.
perhaps also look at implementing some sort of custom commenting engine, and tie both farms into it.
Some things that may assist in this route:
You could look at extracting say the comment engine from the blogengine.net. For that matter, maybe look at a dedicated blogging platform sitting alongside SharePoint like that mentioned above, or WordPress.
I would draw up some cost-benefit scenarios for different methods, including just enabling content approval on the comments in production. This will at least let management understand the level of effort required to meet this requirement, and allow them to make an informed decision on how to move forward.