We have a couple of smaller caps like that on our irrigation pump motors which have vertical mufflers. I can tell you there is a hell of a lot more pressure coming out of an exhaust at idle, and even then the caps kind of bounce and shut. I would think when the owb is a idle it would not even crack it open which would be a smokey situation when you open the door to load the stove. I question if it would even open when the blower kicks on, a 6" exhaust cap is HEAVY!
If you are interested I have a odd cap on my boiler, I guess its a cloverleaf design? I bought it from my dealer, and hawken recommends using it. It is stainless, the top is sheilded to keep water out and heat in, yet the metal is bent to allow smoke to go up. When my owb is at idle it will make ice sikkels on the edge of the cap, so I think thats a sign it is not losing to much heat, if it refreezes the water vapor coming out of the stove. I can try to post a pic if anyone is interested, or I am sure you could see it on hawken's web site.