I agree - then you just use either a Multiple Industry New or one that produces the product you are after to add them to the flatcar (as Bob said, any flatcar that supports the kuid he said should support them.
Due to the way Trainz operates with flatcars and other industry-enabled wagons, it is not possible to load them without using an industry (unless someone knows another way - I know with coaches you can set amount of passengers in Surveyor, but I don't know about wagons (also, only the first item in the list gets shown by default if it does show up when you query a wagon in Surveyor)
Shane