Went to a seminar many moons ago and if I remember correctly Shell said that the amount of lead emitted into the atmosphere world wide per year from Aviation was about 4 tonnes, that is a lump about the size of a coffee table.. Not exactly a lot is it.
At about 1g lead/litre for 100LL, that's only 4 million litres of fuel - 1 million gallons or thereabouts. According to Wikipedia, "The annual U.S. usage of avgas was 186 million US gallons in 2008". I doubt that leaded avgas is the most pressing environmental concern out there, but I think your figures are out.
How much avgas do the military use in drones and trainers? Would it be plausible to wager that it will only be available whilst they still require it?