Are you planning to do this?
I don't currently have the time, although I would love to if I did.
I feel like a corporation setting would cut down on a lot of micromanagement that comes with ordering fleets around. However, that also takes away a good portion of the fun. On the plus side, the corporations could still design their own personal military ships, they would just be limited in number and production. The government wouldn't want these businesses getting too powerful. I always imagined them focusing on commercial shipyards so that they can construct their own freighters, colony ships, fuel harvesters, asteroid miners, or terraformers. Or maybe one corporation specializes in specific ship construction and another focuses on R&D for component designs to produce better ships. They can then sell them to other corporations.
I've got a lot of ideas for it, I just wish I had the time.
That is an interesting Idea, Would you inform the players their opponents are active players on other forums or try to hide it?
It's also a great and logical way to divide a major game, having it all in one thread will be confusing if there are many players involved.
Informing them is what I'm unsure about. Very sensitive information would be openly available to anyone that stumbles upon the other teams forum thread. However, that adds to the fun of it and allows for open diplomacy and espionage.
But I also like the idea you suggested about having a subforum with limited access. I agree that forums are much better than PMs and using that setup would be a huge advantage to any multiplayer game.