Very interesting...
Although, if you threw a bunch of het pieds without markers and a bunch of normals in a bucket could you actually pick them out? It seems that if they were a codom gene you should be able to pick them out from normals, like ybs and spotnoses. As for mixing het pieds and codom genes and being able to pick them out... Could that be because those morphs are already susceptible to pattern changes? I feel the codom het pied morphs listed could easily be altered by the pied gene. I'm not arguing that you can't pick out a pewter het pied, I know you can, but I don't think that alone would make pieds codoms. To me that shows that the pewter pattern is weak and highly susceptible to change via het genes.
If we were to go with this logic every single recessive out there would be a codom morph. Het ghosts tend to be lighter, het clowns tend to be lighter, etc etc. You see what you want to see in a normal looking snake that is het for something.