OK (question about restrictor plate)
 

I've read about cooler vs hotter air for intakes and it makes sense. I have also read that an engine is most efficient at closer to wide open, for less pumping loss due to to throttle plate. Now, if this is the case, could you not restrict how much air goes past the throttle plate, like Nascar 'plate racing', to where your car does indeed make less power and you are spending more time at deeper throttle?

Or would the restricted throttle opening take place of the close butterfly and result in the same mpg or worse?

I think the answer to your last question is yes.

Any restriction of air into the cylinder would equal and increase in pumping losses. I'm not sure if hot intake air counts as a "restriction"...

Hot air would reduce manifold vacuum for the same amount of power produced. Any restriction to incoming air would be the same as an equal restriction due to a throttle plate restriction.

Blocking the radiator so the coolant exiting the radiator was closer to the same temperature as summertime would also help. The wintertime coolant exit (from the radiator) temperatures can be as much as 100 degrees colder than summertime which causes greater losses in combustion chamber heat transfer to the cylinder walls.

In extreme climates with temps well below 0 F some hypermilers can adequately cool the engine with the heater core alone, while getting some decent heat in the cabin.

regards
Mech