Update and resolution.
Somewhere along the line I miscalculated the output of my radiation, measuring it at about 161% of actual. Given my water temp and the real radiation capacity, I was delivering somewhere between 45-50K btuh into the structure during the worst of the cold snap. With a HDD of ~-62.5, that's actually not too bad and much better heat loss than I had anticipated.
My true radiation is 105k btuh, which I could easily hit if I turned the high limit up on the boiler, and which corresponds to a theoretical outside temp of somewhere south of -60f.
So it looks like I have plenty of room with this boiler.