> (I'm ignoring the part about wavelength filtering because it's confusing and would only make your piece of paper heat up less.)
I’m afraid you’re ignoring the interesting bit. You say that 700 W/m^2 doesn’t set things on fire. This is not true. Sure, 700 W/m^2 applied to some target that is allowed to radiate its own blackbody light out to the sky won’t get it very hot, but that’s not what I’m suggesting. I’m suggesting that you insulate the target very well so that its blackbody emissions don’t escape, but you let in the short-wavelength moonlight. Thermodynamics requires that you also let out the short wavelength blackbody emissions, but those are negligible until the target gets very, very hot.
This effect isn’t science fiction — it’s just the greenhouse effect, amplified. Greenhouses (the glass ones and the atmospheric ones) exploit the fact that sunlight doesn’t match the Earth’s blackbody spectrum, so a filter (glass or gaseous) can allow incoming radiation in but trap most outgoing radiation.
In effect, I’m suggesting that a very good greenhouse plus some lenses could get hot enough to start a fire.