So your saying it might be in some obscure local code somewhere. Lets try one. Lets take Standish Me as an example. The building inspector Dan Hill there seems like an fairly intelligent guy but as a local government employee I doubt he has the education or background to write the codes for Standish. He would have to be a subject expert in plumbing, electric, and fire protection just to mention a few. Yet he's the man that does the inspections, how do they do it? A little more digging reveals that the government in Standish realizes that they don't have the expertise to write their own code so they do what every other government does and adopt a code. In this case its the IRC Residential Code 2003. Link below. In this code it does not address the cutting of registers thru floors for the purpose of transferring heat.
Just for reference the IRC code doesn't make stuff up, they use international codes for the mechanical plumbing electric and fire protection. So you see local codes are just adopted national and international code. They can be amended to suit local purposes.
So yea, it might be in some local code and I keep reading on here its a code violation to do it, but have yet to see a reference for it. Might make a good Myth Busters episode.
IRC Residential Code 2003
http://publicecodes.cyberregs.com/icod/irc/2003/
www.
standish.org/public_documents/
standishme_
code/index