Unfortunately, the type of functionality you are talking about is not really what this plugin is designed to do. It was originally designed to work in a premium newsletter type setting where teaser content is offered, but the full content is protected. It does that quite well. Many additional features have been added, mostly driven by user requests. A lot of those have pushed the capabilities of what it can do into new areas. What you are describing may be possible with some customization, but it pushes the boundaries of the plugin's design.
I'm not sure how you have your portfolios constructed, but generally, these are posts that consist of an image file. The plugin wasn't really designed with that type of content filtering in mind, due to the way the excerpts work. It may be possible, depending on how your portfolio is set up, to have the 'more' tag above (if they are individual posts) or some type of derivative of that.
As for the front page, if your front page is a series of blog posts, these should provide excerpts via the 'more' tag. Otherwise, you could use WP's process of defining a static page from your list of pages. You could define that as a registration page or the members page (in 2.3 production version) or in the upcoming 2.5 release, there is a shortcode for creating a login page. That might do it.
Hope those ideas help move you in the right direction.