Whichever method you use, storing these as activities will be a problem since anyone who can view a record can view its associated activities. There isn't a setting you can play with to change that (even changing the OWD for activities to Private only stops others from *editing* the activity).

So although it will be much more work, I think you're faced with either recreating this type of activity as a custom object (not sure how you'd automate the creation of the records) or overriding your detail page with Visualforce than checks for only certain types of activities (perhaps with or without a checkbox) before displaying them.