I've had some success in not-too-bitter winters by shovelling and blowing a thick layer of snow, at least 5 feet or so, on top of the ice as soon as it gets thick enough to support that. This has actually kept at least a bit of water in a liquid state at the very bottom of my pond...but somehow I don't think this will be useful to you...
Seriously though, when I was in Ontario (milder than where I live now, but still far colder than your area), a friend and I experimented with a small above-ground pool in his yard. It was round, 12 feet in diameter and 30 inches deep. We built a wooden frame work around and over it, basically enclosing it in a greenhouse-like structure, glazed with sheets of clear corrugated poly. We didn't actually add a heat source, but the sun was sufficient to warm the interior several degrees over the ambient outdoor temperature, and of course the lack of wind was helpful as well. It was only up for a couple of years before his wife decided she had had enough and wanted her yard back
, but during that period the water had only a few brief periods during which it developed a thin skim of ice some nights, while exposed natural ponds were thickly frozen over.
Maybe you could do a similar, temporary structure, or even a permanent wooden type that could have the clear panels removed for most of the year and then re-installed only during the coldest few months? It could probably be made to be an attractive gazebo- or arbor-type thing, perhaps with enough room inside for seating, etc. If nothing else, it would at least insulate the pond and slow down the loss of thermal energy from any heat sources you may decide to use.
I'll mention that we also tried a solar pool blanket, but it was elevated above the water surface on a wooden framework to allow gas exchange. These blankets definitely work on swimming pools when they are on the surface of the water, but we found that it did very little when not actually contacting the water the way we had it set up.
I have no idea if this is a valid idea in your situation, just throwing it out there for consideration. Looking forward to seeing how you solve your problem. Good luck!