Allow authors to mark some answers non-competing
It may be interesting to see answers that could potentially break the challenge (e.g. built-ins). However, answers that have been banned per meta consensus should not be posted, and answers that are not banned should be competing unless the question's rules state otherwise.
That is, the non-competing status should only apply to the "new languages," unless the challenge defines what "non-competing" means.
The non-competing status may make some challenges more interesting. Using the link in the question as an example, the challenge would seem trivial and boring at first glance without the distinction between competing and non-competing answers, as it contains quite a few submissions with built-ins. However, that is not the case, since the question is nontrivial in many languages.
Including the rule that some solutions are non-competing would signify that the challenge author is still interested in lengthy, nontrivial solutions. Personally, I think "non-competing" should be a stronger form of "allowed but discouraged."
The non-competing status could also serve as a gray-area, in which people could post interesting and nontrivial answers, without the challenge author having to ban them for fairness. I have seen questions (e.g. this) that explicitly disallow certain types of functions, but the banned solutions were still interesting. Allowing non-competing solutions would prevent those answers from never being posted, without losing any "fairness."