So it wouldn't necessarily be a terrible thing to do, then, if you wanted to pick a really amazing, meaty NH and thread it into a family breeding, see what happens... then theoretically 8 gens (or was it 10) you can breed it with your other families as if they were an infusion of RIR because at that point they'd be considered 'pure' again? Have the Germans done anything oddball with them on the other side of the pond... I know the color seems to have developed a little differently, but considering the long separation, pretty cool...
My guess is that it would make a mess that might take you 6 generations to bring back to the RIR Standard. "Pure" is a funny word in chickens. It's not so much genotype but phenotype.
Personally, at my age, I don't have 6 or 8 years to spend on such a project. Just sayin'. Not sure what the end game would be because IIRC the weights on the NH and RIR are virtually identical in the Standard.