Is it possible to use:
restricted = "federation", "rebels", "alliance" It would be very useful if this were possible.
Actually I think you can accomplish this in a similar way (maybe not intended).
There is a restriction in the one deployment at least that requires an engine to get into the area. I would think if you created a special module for your new race your building you should be able to restrict that area to your module. (hopefully the restriction isn’t just generic enough to literally just allow that module type)
I was planning on looking into this for my mod as well.
I was actually thinking of making the Empire more ancient and interesting. Because of this, I wanted them to also mostly only use Empire modules.
It is easy to make a new module that is restricted to Empire only, the trouble is they’ll be able to use all the OTHER modules in the stock game as well.
I made an empire shield, for example that is weak, but recharges very very fast. Alone on a ship it has trade offs. Stacked with the other shields it’s too powerful. I can make the new one “empire only” but I need to also make all the other shields EXCLUDE empire. I can do this but then I have to make a copy of every shield for every race, which is messy.
There is a feature that Cliff may or may not be working on that will allow you to basically disable the original game. Until he gets this finished the only way you can do it is to replace and remake all the modules in the game with new restrictions and remove the original game files.
Actually, JSGME does this already. I can change stock files and it swaps them out, keeping an intact copy.
I’ve made 100+MB mods for SH4, I’m pretty facile with JSGME 
Cliff had mentioned in another post that he was considering building this option into the game for “total conversions.” He didn’t say whether or not he was actually going to start the project or how far along it is/was.
Total conversions are easy, but support on his level would presumably be to permit the game to recognize it for online I guess.