On my D800 when set to AF-S or AF-C, until the camera acquires focus, it won't fire. Indeed the way you describe it seems not possible. Probably Nikon relies more on focus tracking than before.
Thanks for your reply. The way I described trap focus is how it works on my D700. When set to AF-S focus tracking does not occur. If, in fact, this procedure does not work on the D800, that is exactly my gripe.