OP: That's a very difficult question to answer because the physical difficulty depends COMPLETELY on weather/trail conditions, and on the amount of risk you are willing to assume (i.e. how much contingency gear you carry; huts closed = longer bail outs and more water weight, too). There's also the psychological challenge, which you don't ask about and so I won't spend much time addressing, but suffice to say, this is weather-dependent and there really isn't a trail run or ride that compares when conditions are non-ideal.
Some possible scenarios:
-Trails broken out, wind under 50, decent visibility; large puffy coat, insulated pants and Z-rest your only safety gear (so, 15 lb pack): very similar to any trail marathon that has 10K' elevation gain. This represents the ideal case.
-Winds 50-60, visibility not great, some route finding required due to blowing snow, you carry more safety gear because of conditions (maybe 25 lb pack): substantially harder than trail marathon. More like pemi-loop, perhaps, just in terms of physical effort. This represents a more likely scenario, and IMO very few would complete this route in these conditions.
The people I know who have been successful doing this as a single-day route in winter have had the flexibility to wait until the weather is perfect, and I mean Perfect. With trails broken out, not a cloud in the sky, no wind, longish days, and relative warmth, this route is not very difficult. In normal conditions in mid-winter, keeping reasonable safety margins, very few could pull it off. Hope that's helpful!