Well I don't know of anything in literature, but if you are looking for justification you can look to alchemy and astrology. Lead is associated with the planet Saturn. Here is the astrological meaning of Saturn:
Astrologically Saturn is associated with the principles of limitation, restrictions, boundaries, practicality and reality, crystallizing and structures. Saturn governs ambition, career, authority and hierarchy, and conforming social structures.
Saturn/Cronus the Roman/Greek god also was a symbol for Father Time because he brought all things that have a beginning to and end.
http://en.wikipedia.org/wiki/Saturn_%28astrology%29#SaturnWhy link Alchemy and Astrology? Well Silver, one of the 7 Alchemical metals is associated with the Moon. So the link to it's affect on werewolves makes sense.
Lead is a heavy element, all elements above Lead are radioactive and not stable, they all decay into a form of lead. It is the least conductive of electricity of the seven alchemical metals, and in appearance is the dullest. Lead when ingested stores in the bones.
http://skyscript.co.uk/metal6.htmlLead in Alchemy is considered the oldest metal
Basing leads effect off of alchemy and astrology you could say that lead grounds the supernatural. It places limitations and boundaries on them. It weighs them down, binds them to the earth. As it is least conductive of the seven alchemical metals it does not allow the flow of "spooky" energy. It brings and end to those things that have a beginning. If you are immortal then it represents the end, death.
In fact don't use this information, it makes me want to use it for my own writing...LOL
