If a map contest would take place, I'd rather see it as a full fledged map created by the users themselves, map terrain included.
I do not claim I know it all when it comes to making maps, but in order to truly make interesting and original maps the heightmap is just as of a great tool as the objects are.
To make things more "fair" the users competing in this contest should also have the very same objects groups given by the contest holders. That way no one would be able to sparkle the map up with custom meshes, particles and what not that other users do not have access to.
So redders think:
-Heightmap created by users(No limitations)
-Terrainmask created by users(No limitations)
-Objects must be the ones provided
-No custom textures either
That would be the only real contest if you ask me, any other way I can think of now doesn't sound fair or good enough, and making a map contest and only judge by the object placement, would work the same way as judging a diving contest by looking at the swimsuits. I'd see it more as an object placement contest. Having a map contest doesn't sound like a bad idea at all though, and the winner could have their map linked in the news thread and news banner for a week for everyone to download.