Hacky way is to install a plug on your furnace that you can connect directly to a generator or power station. Takes maybe 5$ in parts and is DIYable.
Lots of tutorials on YouTube.
Lots of tutorials on YouTube.
It looks like when it's against code it's "by default": if an appliance is not explicitly listed as acceptable for a plug it has to be hard wired. ND and OK have passed state level amendments allowing it, some places have done this locally, and in many places building inspectors allow it with their discretion.
On a beast of a HVAC system, a simple 15A wall plug isn't going to cut it for powering the whole thing, but if the furnace runs off e.g. natural gas and you just want to power the blower off a 15A plug, that solves the problem of switching power sources.
Eg, in North Dakota
"If manufacturer instructions allow the use of not more than six feet of flexible appliance cord, it shall be permitted on permanently installed one hundred twenty volt twenty amp or less fuel-fired furnaces in residential dwellings."
https://www.law.cornell.edu/regulations/north-dakota/N-D-A-C...