Is this kosher with NEC?
Don't think so.

The suggestion I have seen is put a double pole double throw switch on
the H and N wires and switch them to a male socket to which you attach
an extension cord when necessary.

Any reason it needs to be that complicated? No need for a switch at
all, just put a regular plug on the wires going to the furnace and
move it to either a regular receptacle or an extension cords.



Use a single outlet, not a duplex. And a nice armored plug. Keep everything short and neat.

Bob