I've started doing some reading on NOx scrubbers and apparently alkaline peroxide solutions are good at scrubbing NOx. Maybe I'll test it out by reacting some copper with nitric acid and see how well it works. Peroxide mixed with nitric acid seems to work by turning the NOx into more nitric acid and nitrous acid.
That matches my understanding as well. When you bubble NO2 through water you will eventually reach saturation and rest of NO2 will pass through water. The reason why i suggested adding metal to water is to reduce concentration of nitric acid, hence shift the equilibrium towards enabling you to make more nitric acid from dissolved NO2, same for NaOH... reduce concentration of nitric acid. If you have hydrogen peroxide, that will hellp in conversion of NO2 to nitric acid. May be just use the 3% H2O2 from the store in place of water?
May be sodium percarbonate in water solution is what you need?
I am also thinking you can add a dash of potassium carbonate (cheap on amazon). Potassium carbonate should react with nitric acid, to produce CO2 and Potassium Nitrate, which is poorly soluble in water (242g/L at 20*C) and will eventually precipitate. May be you can monitor how well your filter is working by precipitate of potassium nitrate.