Create new permission - "Change Default Parent"

Essentially the use cases I had envisioned are basically the ones you outline in your patch (certain content types have a hidden parent selection to create a strict hierarchy)

I also think that it might work for my application, however, I still do like the idea of a specific permission, which would allow privileged users to possibly break from the hierarchy if needs be (such as a site-wide editor).

Other uses could also be found, such as locking down a group to a certain sub section of the site while allowing other users to post anywhere. Perhaps a mini collaboration section where no one user would own the parent, but multiple users could attach children to it. In thinking about it now, almost like an organic groups lite, but with much less overhead and configuration needed.