Ken...not sure I understand your statement. If you are asking if the extension method used for the loco roster would be workable if that same method was applied to the cars file, I would find that very workable.? If I could add columns to the cars file, I would be happy.
However, if you are asking if the loco roster method could be applied (technically) to the cars file, I don't have a clue. I am not a programmer nor am I savvy on the JMRI coding.
Paul D