I would like to have a separate tab for "shared documents" where each root tree could be accessed via separate access rights / passwords from non members or read-only members.

That would solve the idea by using PP for procurement or publishing information / documents to people outside the project.

I am already using PP for both procurement and publishing of installation/erection documentation as well as other project information needed on our building site (E.g. safety info/messages). But this is quite complicated with regard to access rights and temporary membership.

We are saving h*ll of a lot of money by using PP for document distribution and I think that many of Your users could do the same. It will also generate money for You as people would share more GB on the PP if this function was provided.