Very interesting discussion here fellows, I'm no aero.engineer but have flown several flying wing designs both model and full scale as with hang-gliders. It would seem that a very low CG is the key component here...notice the NASA wing in photo has a tremendous amount of upward flex which would equate to a high degree of dihedral,right? Also I would wager that yaw control & stability of that design might include thrust vectoring by
varying the individual motor RPMs. Heck they may be also adjusting the thrust ANGLE of motors for pitch stability who knows...anyway I would think an RC model of a true plank design needs some type of gyro-stability system installed.