Hi,
I was searching for an exact way to do this, and am saddened that this wasn't possible (at least 1 year ago).
Like Barbara, I was hoping to avoid the subalbum "private" for protected photos within the album. If I wanted to make changes to my core (which, with your frequent updates, I don't want to do), I would change the Published checkbox to a radio style input with Visible, Invisible, Password Protected and have a login form display if Password protected were checked (or something to this extent). I understand it's not easy, but I just thought it would be cool to have.
The reason I wanted to avoid the private subalbum was because I didn't want to split up my photos into a subdirectory; I wanted a person to be able to see all the photos in one album as an administrator does. Thus, the big alternative is to create the guest [b]user[/b]. The problem lies with that person having access to all "hidden" photos, which is also something I wanted to avoid. I want person A to see these 5 albums and not these other 5 albums. Person B would have access to their own set of albums, person C so on and so forth. If there were a way to create a guest user with full access to specific albums, that would be ideal, like a checkbox.
Anyways, just thought I'd bring this old post up again, wondering if any changes had been made, or perhaps any new suggestions.
Thanks.