Outstanding idea. I just screwed up the courage yesterday to do some box joints with my PC 4216 Dovetail Jig and the parts turned out fair but not perfect. I did have a problem holding the scrap parts to level out the hold down clamps and you have the ideal solution. The only thing I am going to do differently is the screw the two parts together with the vertical part butted against the horizontal part so I can make sure the sight line is even on both ends of the jig. Also make them only 1-2 inches wide thereby giving me more clamping area for longer parts.
When something is advertised as being foolproof there is always a better class of fool that comes along to prove them wrong.