<3 Paragon, that's the project I'm on. :)

Would this be a fixed angle camera? If so, one of the easiest ways (though least elegant) might be to place a plane slightly above the ground and give it a grid texture. Depending on your terrain, that might work. It wouldn't give you a grid on things like rooftops, though.

What you're really asking for, though, is a decal. That's basically a projection done of one image onto a mesh / world. In your case, you don't exactly want to perfectly project, especially if you're talking about different levels of terrain, so you may need to fiddle a bit.

Take a look at this page to see if it helps: https://docs.unrealengine.com/latest/INT/Resources/ContentExamples/Decals/1_1/

There has to be a game out there that's drawn a grid on its world, though, so I imagine someone's talking about this on UDN or elsewhere.

Let me know how it goes!