You would have to be using parallel tenons on the box lid for this to work, correct? In other words, if you are cutting your tenons with a taper from back (buttplate) to front, i.e. matching the forward taper (side to side) of the lid, all of the clamp pressure would be at one end, wouldn't it?
Very interesting, I like the concept A LOT. I cut tapered tenons, so I would have to wedge pieces of folded paper or thin shims in between the vise jaws and the forward portion of the clamps to get a consistent clamp pressure.
Usually I just try to do most of the shaping on the gun, and for finer work, just clamp a big square piece of wood in the vice and then use a big c-clamp to hold the box lid in place on top of it. Of course, then you have to work around the c-clamp which is a pain in the ass.