I was on an overpass, taking a pic of a landfill -- hardly a strategic asset. The question of what I was photographing and why always comes up and then I break into a big grin and go off in rhapsodies about the light, the contrast, the form ...and their eyes soon glaze over as they realize they're dealing with a flake and they let down their guard. But there's got to be an easier way.
I do the same, when a cop is available ahead of time. You're right that it heads off all kinds of problems.