Adding a set of return values at the start (especially after one has written the code) is easy, but as you've noted it's not particularly obvious.

I'd like to point out that the built-in system does exhibit this kind of behaviour in interface towards an optional parameter on Windows and OS/2: you can insert an extra integer parameter in front of the normal command parameter(s). See OS2::Process.

So, it's rarely done, but it is not unique.