Have some key-bind to fire a projectile towards one spot, have the coordinates be registered and then make a marker show up in the player's GUI. The marker should have the distance show above the marker. For an added bonus, maybe you can save the coordinates in a handy GUI whenever you want to come back to that same location.
Maybe we can have this feature be implemented in other add-ons, such as Wrench Events, to be used with triggers.
