Can you get some insulation blown in?

Radiant heat under wood flooring is used all the time, though most suppliers prefer engineered or laminate (a picture of wood) not hardwood.

See link for good explanation.

Here is a link that might be useful: Radiant Heating and Wood Floors

What is "radiant drywall"?

I assume the op was referring to ceiling heating panels from Panelectric or other manufacturer. Wall panels are also available.

Here is a link that might be useful: Radiant Heating Panel Association

I can't imagine anyone would want to pay the premium needed to heat a space with electric heating panels unless it's a summer house.

It's misleading to call prefab panels on a ceiling or electric wires behind, in front of, or between drywall panels "radiant drywall". Electric radiant heating embedded in finish materials has been around for at least 50 years that I know of and doesn't need new name.

You probably would be better off with a radiant ceiling heating system. When insulated above it will draw the heat down and it will heat everything in the room. If you go to www.calorique.com you can see an install manual of radiant ceiling heat which is very easy to install. Regarding the wires in the drywall, this is a very old system but well worth it when it was installed. They are now failing and folks are looking to replace them with another radiant ceiling heat product.