So this has been up, but not very well known, for a while. To get the ball rolling and do some follow-up'ing on this I'm making this topic.
Through some quite awesome reverse-engineering work by Eximius, we have enabled shared-armies for Forged Alliance.
The binary modification and corresponding mod can be downloaded from here: http://faf-shared-armies.tk/
The UI-mod currently allows you to share your army with allies, and switch control to allies who have shared their army with you. Ideally, this would be integrated as a lobby option so coop-1v1's can be initiated directly. That requires a bit more work though.
For the technically inclined
It works by hooking the engine function "LoadSavedGame", injecting some code that modifies an internal list of allowed command sources for the given army. The call is sim-side, and thus is called through a sim-callback, where the proper cheating validations are made.
Any ideas/sketches for how the lobby UI should work with this would be greatly apreciated. Would be nice if Xinnony himself would like to contribute
Have fun killing cooperatively.