Is the Torii in front of Atago bronze? It looks like it is, and I’m wondering if that might be something of a rarity?

Good eye.

Bronze torii are indeed fairly rare. Wooden torii are a dime a dozen, and if you spend a lot of time around shrines you’ll see a lot of stone torii as well. Bronze on the other hand? I went back through my photo collection and was only able to find three bronze Torii (besides Atago of course)

One found in front of Takachiho Shrine in Miyazaki, which I posted about here.

http://somethingweirdisawtoday.tumblr.com/post/162683954056/torii-at-takachiho-shrine

This one, near the main gate of Toshogu Shrine, in Tochigi.

And last, the largest bronze torii gate in Japan, the Daiichi Torii at Yasukuni Shrine in Tokyo. (Sorry about the terrible photo)