if one of you guys is messing with this, another feature request *g*:
because silverstripe doesn't have url-namespaces between controller/action/director-rules and page-names, it would have to check that you can't e.g. create a page with URL "admin" and a subpage "security". in both our current approach and any sub-page additions, you'd probably want to compare the first url-level against classnames in the manifest and custom director-rules.