BUMP
I think this is still a very important feature for Multisites. In the User Administration module, one should be able to dynamically add the starting point(s) in the page tree for the user. Has there been any progress in the meantime? I would love to contribute, but I have no clue where to start as I am not really into Flow.
By the way, it also happened to me that the NodeTreePrivilege only grants or revokes WRITE access no nodes … sites and nodes are still visible in the page tree or Content module.
This is a related thread: Project proposal: Introduce parameterized roles