[motto]

I don't know that parkerising would be a success in the application you have in mind Roberta. My understanding of it is that it's porous and requires oiling to make it effective. It seems mostly to be used as a small arms finish and is therefore 'maintained' which would not be the case with vehicle components.
Possibly parkerising was not more widely used because of its short comings or quite likely because it required the use of manganese or more specifically manganese dioxide which I believe was in short supply during the war. In other words there may be cheaper more durable finishes. Tin plating is quite successful on small parts, it's non toxic (unlike cadmium) and doesn't cause embrittlement (unlike cadmium). It just doesn't look military and needs to be painted over which you may as well have done in the first place.
That's the limit of my knowledge. Perhaps someone else can add more.

David