My thanks and congratulations to Serdar Bilici for hitting on this idea and so thoroughly testing it. His step tablets speak for themselves.
I have never had to go down the road of paper pre-acidification, because I only use papers that do not contain chalk (calcium carbonate), such as Buxton and Herschel.
But I can see that this work does greatly extend the usefulness of my argyrotype and new cyanotype (and probably other siderotype) processes to a greater range of presently-available papers.
BTW, sulphamic acid can also be used for the first, acidic bath in new cyanotype, if nitric or hydrochloric acids are, for any reason, unacceptable.