Another road test candidate for air bags....
So you're saying that in your vast experience working in a fireplace mfrs's R&D test lab, you don't foresee a problem?
It is an unknown quantity unless tested and listed. Therefore, it would be in violation of every major model building code, fire code and nationally recognized standard. It you suffered a loss after being warned here, your insurnace company may be able to deny your claim based upon gross negligence on your part. Should somebody else get hurt, you may not only be personally liable but criminal charges possibly could apply. Regardless, if not tested and listed, then the warranty and listing of the appliance is null and void. If you went to sell the house and failed to disclose this defect, you could be in big trouble should someone down the road get hurt or suffer damages.
Why is it everyone thinks they are an expert in fireplaces? Gee, let's see, if I can somehow adapt pipe A to pipe B then how could be be a problem? Just for starters, try fire, carbon monoxide poisoning, odors, soot damage delayed ignition, oxygen starvation, flame proving dropout, etc.