I find the whole discussion interesting too, Chris. Since there have always been discussions about it being more caused by iron in the paper, a mold, a combination of the 2.
I think the Da Vinci piece, because it is drawn, is of concern to scientists, too, since washing, buffering, bleaching could cause the image to literally wash away.
I've read where older movie posters (vs glossy modern, that seem to resist foxing) were treated successfully with a wash/buffer/bleach bath. It lessened or removed the rust colored spots and the paper overall was more pH-balanced, as well.