I doubt any competent HVAC contractor would risk their plumbing license on a code-violating hack. You might be able to get an inspector to sign off on the variance if you can give them the analysis from a registered PE that shows it's not really a hazard. You'll spend a lot more on engineering and contractor time than it's worth though, IMHO.

Think seriously about installing a heat pump water heater and just turning off the oil boiler when not being used for space heating.