I am not familiar with any of the particular brands you mentioned, but I can tell you from our own experience: The attorneys who represent these companies say, DON'T ENDORSE ANYTHING THAT YOU HAVE NOT ACTUALLY TESTED. The say that because they are risk-averse and do not want to expose the company to unnecessary liability.

But that does not mean that what you have in mind is a safety risk. In our case, we installed a Direct Vent system into our prefab fireplace made by a different manufacturer. And it has been okay, in fact, it's been terrific. So you have to use a bit of common sense. If the thing you want to install into your fireplace fits easily and does not affect the area around the firebox that is made of noncombustible material, and if the installer says it's okay after he or she has looked over the job, it will probably be okay. It helps to educate yourself a little bit on how these things work -- where the air that feeds the combustion comes from, where the fumes are exhausted, and the "clearance" around the firebox (the distance between the components that will get real hot and the components that are combustible). If you are satisfied with these specs, or you feel comfortable after discussing these things with an installer, you should be okay.