Hmmm... I don't think it's nUI. I can't bind a mouse click in the standard Blizzard key binding window either. nUI does not alter that in any way how that window works, so even the Blizzard key binding window is not allowing mouse click key binds.
What people don't get is that I am, ultimately, an artist at heart. My brush has two colors, 1 and 0, and my canvas is made of silicon.