"Lead rot" is the lead content of the casting alloy oxidizing. Lead "rust", in other words. Metal castings are actually quite porous, and will take on moisture which produces the "rot".
You can treat it by going over the whole figure with a small amount of mineral oil on a soft cloth or cotton swab. Let the figures sit for a while after treatment, then wipe off any mineral oil that hasn't been absorbed. Luckily, homecastings are solid metal and can take a lot of oxidization without permanent damage. The old Britains hollowcasts are an uneven shell, paper thin in places, and they can simply crumble if the "rot" is internal.
The mineral oil treatment won't repair damage, but it'll stop moisture being absorbed by the castings in the future. This treatment will also revitalize the paint, which on older homecasts is probably an oil-based gloss enamel.