This AddOn makes the hit boxes of nameplates you choose to show visually distinct.  See the images for previews.

When you choose to show nameplates there is a fixed area that catches mouse clicks for each nameplate.

How does it work?

The AddOn asks to be notified when nameplates are shown.  When a nameplate is created for the first time it is assigned a backdrop image.  When the nameplate is shown the AddOn sets the alpha channel on the backdrop to 1 to make it visible.

Why doesn't it show for friendly players in dungeons or raids?

This is due to a general restriction added by the World of Warcraft developers.