I see no reason that a good proposal and example implementation wouldn't be accepted for addition to go. The Either/Maybe monad is so powerful and is incredibly straight forward to use. So the argument from a language user perspective is already won, it's makes the intention of code much clearer and gives the type checker massive help in verifying that your intention is the only possibility at runtime.

I expect the implementation and grammar/syntax addition would be the most difficult as that seems to be what one of the main focuses was during Go's infancy, make the language as easy as possible to parse/lex.