Those all seem like very good ideas. My head will always start steering me into the overly-engineered types of solutions causing me to miss the easy button at times, so my idea for a custom course activity may be over the top.

My other immediate thought was the assignment module too, but see that you covered that. It seemed like a natural place to do what you need. I am actually not currently familiar with report sources dealing with assignment module, but it seems that since one does exist perhaps it would be a logical place to have a new feature request to extend this report to allow for content restrictions based on direct reports.

As for the notes idea, perhaps that could also work if some extra capability checks were built in to it for the manager role.