Unfortunately, we have never seen Nick's naked ass in all its glory. He has done nude scenes, yes, but there's always been an obstacle: or it is filmed by one side, so you can't see his crack, or the actress strategically grabs him with her legs, so you can't see anything, or he's shot from a distance and there is a fence and he's wearing a shirt, etc, etc, etc. Or, like in the photoshoot you mention,he only shows the "beginning" of his butt, but not all the crack and cheeks. So, as far as I know (and I do hope to be wrong) we have never seen Nick's naked ass ENTIRELY AND CLEARLY and, by this time, I am afraid that we will never get to see it, either...
Oh, Nick, why do you torture your fans like this?