Just how transparent does it need to be? At first I was thinking about those mylar balloons that can hold their helium for a few weeks. Usually one side is reflective and mirrored, and you can see through from the other side, sort of like a darkly tinted window. I’m pretty sure the mirrored part is conductive.
What shape does it meed to be? I’ve got your answer. If it can be used in sheets or panels, it’s a piece of cake - almost.
I’m thinking sandwich. Find your plastic that’s transparent and fits your requirements for temperature range. Polycarbonate? Acrylic? I don’t really know if they’ll make it to 300°F. Second thought, just use glass.
Then sandwich in between the plastic or glass layers a liquid layer doped with enough ions or electrolyte to carry current, and then tweak the solution to obtain the working temperature range. This might be the toughest part.
Some glycol / alcohol mixture might get you down to -50°F without freezing, but I’m unsure if it would be polar enough to keep enough ions in solution reach the 300°F mark. Maybe some strong acids would work better.
PCBs used to be a commom additave in transformer oil to raise its heat capacity, but they don’t dissolve in water very well at all. You’d have to use a non-polar solvent for those. Pentane? Hexane? They’d hold PCBs in solution, but we’re back at the freezing point/boiilng point problem again.
Maybe if you could get the freezing point spec worked out chemically, you could conquer the boiling point spec using physical/mechanical means, i.e., pressure.
The radiator in your car keeps the coolant under pressure so that it can take a much higher temperature without boiling. So, make the glass or plastic sheets structurally strong enough to withstand some pressure, maybe just by brute force with the solid layers ~100 times thicker than the liquid layer.
Just how much current do you want to carry? Whole, whopping amps to produce some heat? Or would pico amps work, like for carrying a signal that can be detected? If miniscule amps will do the trick, I’d just go for plain old wires. Pentium III class CPUs use “wires” inside that are only 0.18 microns wide. That is small. A red blood cell is about 40 times as wide. And you need microscopes to see them.
I’ll quit rambling and let others have a chance.
I want 10% of all the money you make from whatever product you derive from my great ideas. I’ll have my people contact your people; blah, blah, blah…