It ain't in the proposed treaty. This is in the proposed treaty
Currently, there is no official draft of the treaty. That is what the past week in New York was about, to try and get a working draft that can be used to generate an actual treaty in the 2012 meetings. The "draft" on the UN website has been changed several times.
Keep in mind, the treaty referenced in the Senate letter is the Arms Trade Treaty, not the Inter-American Treaty