Can I use the API to identify members-only pages? It is not in siteStructure() - is it anywhere else?
And if not, could it be added to the siteStructure() object?
Are you trying to identify pages that are marked as “members only”? There currently isn’t any way to get this information using the API. Feel free to submit a feature request here for that.
BTW - Don’t forget that in additition to using members-only and password-protection for pages, it’s also possible to check permissions using code which won’t be available in any site information.