Nae dice :(

public class RoadWorldGenerator implements IWorldGenerator
{
    @Override
    public void generate(Random random, int chunkX, int chunkZ, World world, IChunkGenerator chunkGenerator, IChunkProvider chunkProvider)
    {
        int blockX = chunkX * 16;
        int blockZ = chunkZ * 16;

        BlockPos topBlock = world.getTopSolidOrLiquidBlock(new BlockPos(blockX, 90, blockZ));
        System.out.println("Top block: " + topBlock.getX() + ", " + topBlock.getY() + ", " + topBlock.getZ());
        Chunk chunk = chunkGenerator.provideChunk(chunkX, chunkZ); // Tried using both chunkX/Z and blockX/Z
        chunk.setBlockState(topBlock, Blocks.NETHER_BRICK.getDefaultState());
    }
}