Moved this to the WAM section (although it fits both WAM and Form Builder).
One thing people forget to realize here is that the Form Builder is not a typical PHP built form setup, it's a flash form. I don't know if the future build will include any possibility for this, but to be honest I don't expect it will as it would take a lot of coding to assure that the form was functioning with the different types of servers, as well as making sure databases are setup etc. Not sure if CC will delve that far into it or not since right now it is the responsibility of the web designer to see that these functions are setup correctly.
Login situations need extensive amounts of PHP and a few other things to function correctly as well as good security coding and I don't see CC doing this as most of their motto is to make things clean, simple to use, very functional and inexpensive. When you start getting into Login stuff, you're dealing with more of a CMS system and somewhere along the line I could swear I saw them say they wouldn't be developing something like that (or I'm imagining things which could be too lol).
As for the Website Access Manager, it has absolutely no connection to your website itself other than the access to pages, files or directories. Once that access has been used it is no longer resident on the site. It is not a viable way of making login pages. It is not able to store cookies for you or anything else that would resemble a login situation. It is only to allow or disallow access to pages, files and directories. There is no control over the login box that pops up to the user trying to get access, that is all part of the WWW and is created outside of anything we have control over.
I hope that makes sense, and sorry to burst your bubble on your request, but it's not possible to do with any of CC's software. You are definitely able to do this with the use of PHP though, but it's something you would need to do on your own, and I'm not sure if the CC Form builder in it's current form will be able to be used for this, you may need to look outside of the software for this.