It's interesting that some people will "destroy" the right brake lever just to have a purist looking FG bike.

I agree with the OP's decision to run a rear brake. More stopping power is always good. If you choose not to use it, disregard the right lever. As for weight penalty, lots of people here are running 20lbs entry level FG steel bikes, so unless you have a 13lbs bike, I don't see brake calipers as being a weight penalty.