I did implement this myself on my installation and works quite well, especially when you allow others to log in and view as guest and they open every single tree.
One thing that does have to happen I know before this feature would work is that each tree has to be closed at least one time under each user, for the collapse to work, since they do not populate the sql database with a tree-state until this has happened, but it would still be a good feature, maybe you can add something to where when you add a graph it populates that table on creation, and when a new user is added all are populated for them too, I know the last part would not be fun to implement so I think most would be happy with just Expand/Collapse all trees.
