OpenRA

The simple answer is that we don't have access to the implementation details of the original scripting formats
Are the documents linked from the Reverse engineering page of the wiki not sufficient? There must be some sort of access to the original scripts available otherwise how were any missions ...

I'm also interested in the single player.

I had a quick look at how OpenRA implements mission scripting to understand why it's so unpopular to add new ones.

Two questions for any developers in the attendance jumped into my mind right away:
Why doesn't the OpenRA engine support the original format ...