Without my plugin there should still be two radio buttons to choose from for readability settings. Does your method of displaying aggregated posts work as you want if you select the second visibility setting? I suspect it won’t.
At any rate, you are describing how my plugin is supposed to work. If a blog admin only wants their own users to see their content, then that content should only be available on the site the user is a member. The content should not appear elsewhere in the Network or indexed by search engines.
Of course you can get around the blog_public setting by editing whatever script you are using to pull/push/aggregate content to the parent site. That code is abiding the privacy setting of the blog is all. So fiddle with the code you are aggregating with so it does not (might not be an easy tinker though).
Whatever code you are using to aggregate, there may be pitfalls once you commence ‘atinkerin’. For example, the blog_public option by default is either a 1 or a 0. Basically, by default the visibility setting is either to allow other scripts or search engines to index the site’s content, or not. The additional options added by my plugin merely add additional values less than 0; -1, -2, -3. But the basic functionality of the reading settings remains – index content or not. SO basically any value not equal to (or less than) 1 will not be indexed. However, plugins that aggregate may only look for values of the blog_public setting is equal to 0, rather than if the setting is less than 1. Know what I mean? Test, test, test.
BTW, each site has the privacy setting, so whether or not SuperAdmin is logged in creating content matters nothing in this context. If the site is marked “Users Only” the only way the content can be seen is on that site by those users listed as members of that site.
