Russ - I think you're not on the right track here.
First, putting the owner's
name on the reefer was not outlawed, it was the size - Swift used to use huge S W I F T letters that were like 6'+ in size, covering the entire side of the car. The ICC ruled that lettering had to be 12" high or less for a car to be used in interchange service from 1937 on. (Of course ICC could only regulate interstate commerce like interchanging of railcars - if you owned say a logging railroad and your cars never left your own rail lines you could letter them however you wanted to.)
Second, I'm not aware of any instances of railroads selling advertising space on their freight cars !! If a reefer said "Baby Ruth" on it, it was owned or (much more likely) leased to the candy company, either for hauling the finished candy or hauling...well I don't know, sugar or corn syrup or something. Usually companies leased the cars from a leasing company like U.R.T. but sometimes they leased them from the railroads - which is why you will sometimes see pics of old cars with a railroad name and reporting marks on it but with a private company name on the car also.
They weren't called "billboard reefers" because the railroads sold the advertising space on the sides like a outdoor billboard-sign company would, but because the lettering was HUGE and very eye-catching like on an advertising billboard.