I'm helping a family member set up a site which will need to include a secure client portal, providing access to documents and a simple database. I have been encouraging them to go with a more established, open source CSM like drupal or joomla, whose capability in this area is evident. However, they have a strong preference for Squarespace. Does any one know if it would be possible to accomplish this with the new developer platform for squarespace 6?

I've spent well over an hour searching google, the squarespace site and stackexchange, but can't seem to find any clear answer to this question. I'm grateful for any insight you all can provide.