Re: What's the mechanism for adding new error codes?

http://www.ietf.org/internet-drafts/draft-ietf-tls-http-upgrade-04.txt

is awaiting IESG  announcement of an Internet-wide last call -- it 
was LC'd at TLS-WG and here.  It will establish a new IANA registry 
for new reply codes.

Policies, forms, etc. can be found there...

Thanks,
Rohit

PS. I'm conspicuously ducking the question of whether your intended 
application is "correct" or not; this is just an answer to the 
question of "how"...