Found this searching "box wing airplane" from 2005:
http://tinypic.com/1ta9lv
so there's probably other examples to indicate that it's been thought about for a long time. A big reason this wouldn't be too attractive for the type of airplane shown in this picture (an airliner with a diamond/joined wing/tail) is that it would probably be much more involved to add fuselage plugs to generate a long-fuselage version, or, use the same (or minimally modified) fuselage with an "improved" wing. For example, the more recent versions of the 737 and 747 probably re-used a LOT of the existing fuselage design and tooling while incorporating new wings. But for a single-point design like the Synergy where if it was a kit, it would be available in the "one size" for a couple hundred units, until the "next size" becomes available. If the performance advantages prove to be worth whatever the cost, building, and/or storage compromises might prove to be, then it probably would find some customers... At least give us something new to see at fly-ins... Though if I were going to design a first iteration to market, it would be the 2-seater. This, by far, seems to be the most popular seating capacity in homebuilt airplanes. More seats than this starts to get very expensive to build and power, and single seaters are always a very limited market (and not much less cost to build or power than a 2-seater).