Things like /account/ will actually be matched to a repository since it matches (?P<repo_slug>[^/]+), so either:
Use a new URL with / in the url, e.g. /account/settings/, or
Force the match of this URL prior to the CNAME resolve. For this to work, we'd need to keep a static list of URLs that aren't repositories. This also means that people can't register a repository named e.g. 'account' and still use the CNAME feature, so option 1 is favored.