Composite materials might not be able to accept the heat or conduct it well enough to radiate it to the room effectively. Find out what the conductivity and emissivity of the decking is and ask what materials the wiring can be placed directly against. Being able to disconnect the cables may not be allowed by the electrical code.

Heat of this nature is first conducted through the decking material and then radiated by the face of that material through the air of the room to objects in the room. Of course, the flooring also heats the room by convection.

Insulation added below the decking will help to reduce heat loss downward and increase heat movement upward but if you want to also use a radiant barrier to help with that effect (I doubt that it does much at that temperature) at least a 3/4" air space is necessary since otherwise the energy transferred will be by conduction through the foil and the other materials in contact with it instead of by radiation and there will be no benefit. In a house, radiant heat can only exist in air. Therefore, it is not the heating cables that provide "radiant" energy, it is the warm surface of the flooring/decking where it is in contact with air.

I suspect increasing the thickness of the extruded polystyrene foam board (Styrofoam) would be just as effective if not better.

Don't burn the house down to save energy.

If your room is enclosed you may want to consider heat panels as a way to heat it.
We installed the panels in rooms that are in DS's horse barn 2 years ago. The tack room is 14x28' and 2 of the panels supply more heat than is needed. They can be connected to thermostats or timers, but I recommend you use thermostat to keep the room at a set temp.

I know this isn't an answer to your question, but I thought it may work for what you're needing.

I have no affiliation with this company............

Here is a link that might be useful: e-heat

Thanks.

How would I find out what the "conductivity and emissivity of the decking is" without conducting my own lab tests?

They put (certain kinds of) heating wires under carpet and laminate, so I think they would work. The composite has exterior quality durabilty.

At any rate, I'm starting with hollow core decking. Maybe I can even put PEX tubing through them for hydro-radiant. It would probably work much better than wires anyhow.

It's all glass 4 sides to the floor (Mounted to house at picture windows) and roof, so no room for vertical panels.

The emissivity of the composite material is probably similar to wood and/or plastics which is probably good enough; the larger issue is how well it conducts heat if it is hollow since it would radiate infra-red energy from all surfaces.

The best commonly available radiator of infrared energy is a thick conductive metal with a large surface that has a dull finish, like cast iron which works well even with (non-reflective) paint on it. The worst would be polished aluminum.