Well, I don't know if this will be understood very well because it appears to be correct but my experience with rocket's tells me otherwise. I have not gotten around to doing this myself but working with rocket stabilization all the time keeps me from building a quad like this.
But! Because the quads can be built this way very easily I'd like to explain WHY I think you guy's build your quads UPSIDE DOWN! :)
The CG is supposed to be over the CP! In other words if you put all the weight ON TOP of the rotors these craft SHOULD BE MORE STABLE!
I've had a hard time proving to even some so called rocket scientist (Like the ARCA GLXP Team) you cant put the CG below the CP and get stable flight without a lot of control input. It's just harder!
Imagine a seal balancing a ball on it's noze. The amount of correction needed is very small. Now with a ball hanging from a string the amount of correction needed is much greater to balance the ball on a point.
I think the reason we still put the CG below the CP is because it looks right and helicopters pretty much have to work that way but quads DON'T!
So how about trying my theory out? :)
If you notice the Curiosity Mars Rover for example the rocket engines are BELOW the CG like they should be.
Quad rotors will be more stable with the CG on TOP!