Just to follow up: I was fiddling with a new board and thread. When I edit the thread, there's an option to set the access level, but it seems to go back to whatever the setting is at the board level when I open the thread for editing again.

If a person wanted to have two threads - one with a higher access and one perhaps at "0" - do you think these threads should be created from different boards? I wouldn't want the setting for one thread to override the other thread, but I don't have experience with how this works in practice. My threads have always been set to "0" in the past.