Hi Martin.
There was a GTD forum, but, it had very little traffic. Basically a month without a post. And, even then those posts weren't specific to a GTD really, they were common to all space frame GT40 replicas using a Renault gear box. I took the posts and assigned them to their respective categories so no information was lost.
If a marquee has an active company and someone on the forum who answers question I think the make specific forums are a good thing. The RF forum started that years ago, and the MDA, RCR, and Superformance forums are active now with factory support and active company representatives. I'm not sure what to do with the Roaring Forties forum as they don't seem to have active company support, but, there are a lot of posts there and a lot of information that is actually specific to that car. The GT40 Australia forum isn't too active and only has about 30 threads in about 2.5 years. The most active threads in there are build threads and they might should be dispersed to the Builders Forum.
In short, I'm not sure Southern GT will have one or not. You are the first person that has brought it up and I've not had any contact from the Southern GT folks about one.
Best Regards,
Ron