Very interesting problem!
If you're dealing with an existing structure which has (past-tense) already survived for quite some time, the Canadian Building Digest's CBD-230 may be your answer. It is essentially a guideline on how to rationally argue that your building is "field tested", assuming an undeteriorated condition of the structural system. Available online at: irc.nrc-cnrc.gc.ca/pubs/cbd/cbd230_e.html
Now, you COULD argue along similar lines regarding the performance of such a system given the long, well extablished, record of performance of these buildings on the stepps. However any building official who questions you automatically wins as you will have no real evidence of the long term in-service behaviour without doing alot more research.
Good luck!
YS
P.S. Love a challenge, and this would be great. That said, and along the lines of akastud's comment, I'd be charging by the hour!
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...