All you really need is the internal structure which isn't going to be all that secret anyway. Characters have stats, those stats are access by function X etc. If there was decent documentation on the functions themselves, you could share every function definition and the comments on what it does without anyone needing to see the actual code definition of said function (at least to code new stuff, making bug fixes is another matter).

Plenty of apps offer API as well, which would probably be more work, but could get you similar results in an even more controlled way.

There is also a fallacy that the source code for CF needs to be secret. The Imm staff has always wanted it to be secret, but that doesn't mean it has to be in order for CF to be successful. It doesn't mean that if it wasn't secret that CF would be less successful. Those might be true, but it isn't a given, no matter how much you assume it is.