The issue is that the missing.php script (which e.g. causes the redirect from https://vec.wikiquote.org/wiki/Bla to a page on Incubator) does obviously not provide the same function for closed wikis, for which people should contribute on Incubator.
Is it possible to create e.g. an option to normally redirect people to Incubator when they access such a closed wiki (i.e. a closed wiki whose URL would be redirected to Incubator by missing.php if it did not exist at all; so excluding closed chapter wikis etc.), and to not redirect them e.g. using a parameter like ?incubator=no in the URL?