I have successfully restricted a user group that my client will be part of from creating certain types of pages using canCreate(), so now there are only 2 page types left (the 2 they will actually need) - Page and ArticlePage (as per the tutorial).
The problem is, one can create an ArticlePage as a child of ANY page, not just the news page (ArticleHolder). Is there any way to restrict ArticlePage to only be able to be created as a child of ArticleHolder - pretty much the reverse of allowed_children, like a "not allowed children" type concept, to specifically disallow the creation of ArticlePages as children of regular Pages?