i think that the key will be to not try to hide at all what they are. it would be best to treat them in the tank just the same as they are actually used--either stacked up or laid down as pavement.
i have seen some real cool marine tanks adorned with bottles, net floats, toilet seats and other jetsam and garbage. this concept can be quite successful. however, other similar ideas, such as wrecked model airplanes or boats that are completely out of scale with everything else, don't work at all.
it would be worthwhile to take the extra time to find real old antique terra cotta bricks.