ecto_ros contains publishers, subscribers, bag readers and bag writers for many common ROS message types. You can place subscribers as the inputs to your graph and publishers at the outputs, using converter cells as necessary (for instance, to get from Image messages to cv::Mat objects).
ROS topic remapping works much like it does anywhere else. Ecto scripts that contain ROS nodes will contain the line:
ecto_ros.init(sys.argv, "extract_largest_cluster")
which will do remapping as expect in ROSland. So an example of commandline remapping would be:
./my_script.py /input:=/camera/rgb/image