With all due respect, would you mind saying why?  Please keep in mind the following, from my perspective as someone who is at the same time (a) not a member of LangCom, but (b) very active in managing Incubator on a day-to-day basis.

• In doing this, nobody would be changing the requirement that a language must have an ISO 639–3 code in order to be approved (or in order to have a test on Incubator). The question of the existence of the code and question of the use of the code in our projects do not have to be the same question.
• I see the Mapuche test projects as having been pretty inactive for a while. I don't know if that's simply because there is no interest in the native-speaker community now, or because the community finds the idea of editing under this code offensive.
• It's not for me to decide whether the community should find the code offensive.  Nor am I taking sides as to whether anybody is or is not racist, or is or is not perpetuating anything inappropriate. I'm simply interested in running an experiment to see whether the community would be willing to get more involved on Incubator under a different code. Because, like it or not, evidently the community itself finds the code offensive.  
• It's easy enough to test the idea in Incubator now without messing anything else up. I do this by creating redirects on the wiki from all the W?/arn pages to W?/qmp pages. Nothing else would change for present.  Nobody would even have to change any underlying master redirects yet: 
  • arn.wikipedia.org (presumably) redirects to incubator.wikimedia.org/wiki/Wp/arn, which would then redirect using the wiki markup to incubator.wikimedia.org/wiki/Wp/qpm