This is a good idea, but I think a comment system on the report itself would work better. Would make it more streamlined and not add 1 or more SQL queries when reports are created, closed and deleted to create, close (lock) or delete the report discussion topic.
Report comments could be implement either like the profile comments mod or as threaded comments; eg:
Hope that layout made sense.
Always use the Support Board for phpBB support. One PM answers one. One post answers many.