Nikon changed their implementation of the AF-ON button in the Nikon D600, D7100 and other new Nikons (including, I believe, the D800 and D4). The change made it impossible to setup a focus trap. Here is a workaround that may be usable for some applications.
On professional Nikon DSLRs, there is a dedicated AF-ON button. The AF-ON button activates the autofocus without you having to half-press the shutter. Among other uses, the AF-ON function can be used together with continuous AF to give you total control of when to start and stop autofocus (making focus-and-recompose possible even with continuous AF). More information here: Back-Button Focusing: What, Why and How?
About Focus Traps
A focus trap is a technique for capturing a subject in focus. You first turn on "focus priority," which means the camera will not release the shutter unless the subject under the AF point is in focus. Then you use AF-On to focus on a preselected target area, while the subject is still away and out of focus. You then hold down the shutter but because the subject is not yet in focus, the shutter doesn't release. As the subject comes into the target area, the subject becomes in-focus and the shutter is released at the moment the subject comes into focus. It is useful for capturing fast-moving subjects. More information here: Swing into focus
The New AF-ON
Nikon changed the way AF-On works in the new Nikon DSLRs. Previously, when focus priority was active and you used the AF-ON option, the shutter would not release until the object under the AF point is in focus. Now, if you use AF-On and you press the shutter, the shutter will release whether or not the subject is in focus, even if focus priority is activated, thus making a focus trap impossible.
1. Activate the AF-On option (on the D600 and D7100, assign the AE-L or other customizable buttons to AF-ON. This will automatically disable the shutter's half-press autofocus).
2. Select focus priority.
3. Move the AF point to the desired area.
4. When the subject is near the target area, simultaneously hold down the AF-On button and the shutter (all the way, not just half-pressed). While the camera is still focusing, the shutter will not release. In other words, it will only release when the subject is in focus. Again, for this to work, you must be holding down AF-ON, and focus has not yet been acquired.