Is there any way of adding user access control to the Pages within ZenPage, so a user must log in to se the page content, like it is possible in the image gallery view (through the gallery editor)

There is no way, but you can have unpublished pages or articles that you only can view if you know the url. Maybe that will be added sometime later but not in the near future since before adding new features like that a few internals of Zenpage will be reworked.