This is just what we did. We created a master-project with projects as tasks on the first sub-level (elaborated deeper on lower sublevels, as desired). Every project manager reported for his project/task in this master project.
The only problem we had to overcome was that GP does not (or does it?) control the number of copies that can be opened from the same file. So, every project manager could in principle open-edit-save his copy and override what others may have saved before. A master project with three-or-so projects should be manageable, but we have more than 20 joined into one, which would be a nightmare.
We solved this by placing our master project in our document management system, which prevents opening multiple copies for editing and it works fine.
Hope this helps a little.
And again, thank you for this excellent SW, Dmitry. Молодец!!!