I had to do this today for an internal application, but only one extra form in my use case. It is the only way that I am aware of, but lets think out of the box.
If you made a connected table with just the active projects within the same application, and used that as your relationship that is selected in grid edits, you would have isolated a list of data for selection. Perhaps you could then use a formula to relate the child to the parent projects table of Active and Non active data, so that you retain a relationship to the project once it is no longer active. That is a way to solve for this, but in my opinion this takes just as much management as the multiple forms and assignments to grid edits.
