From the description quoted, it doesn't sound like nitric acid, but silver nitrate solution used to impregnate the paper -- and I wonder if it wouldn't be possible to sensitize that in solution rather than with the hydrogen-rich gas. If I'm translating the old chemical names straight, it looks like he's reacting a phosphine with the silver nitrate, producing a silver phosphide, silver phosphate, silver perphosphate, or silver hydrogen phospate, which could probably be done by treating the paper with an appropriate solution of the correct salt or acid (phosphoric acid would be the obvious first one to try). Reaction with iodine would then produce an iodophosphide or iodophosphate -- which indeed ought to be light sensitive just as a halide would be, possibly with greatly extended spectral response (the phosphorus ought to weaken the ionic bond to the silver, requiring less photon energy to form a developable latent image speck).
No nitrocellulose (because no nitric acid, and in any case very little nitrocellulose would be formed in short exposure and in the absence of sulfuric acid as a catalyst and dessicant), no hydrogen gas if you could react in liquid -- but without an analysis of his paper, it's impossible to be certain what salt or complex he was starting with before sensitizing with iodine vapor. It appears he was originally trying to produce a Daguerreotype on a paper substrate, which would have been a revolutionary advance.
Anyone here enough of a chemo-historian to translate the chemical names into modern IUPAC designations?