Everything I’ve read has said that mid- or late-spring is the best time to collect trees for Yamadori. However... has anyone ever tried to collect in mid- or late-winter?
Seems to my novice mind that the only issue here would be frozen ground. A pick-mattuck works just fine to chop out the root block, and as long as the trees are kept outside until Spring anyway, keeping everything dormant/frozen... would there be any negative impact on the tree?
This is flawed thinking and misunderstands how collection and trees work. It works roughly like this--in winter trees store reserves in their roots. In early to mid spring, those reserves are pushed back up into the tree's upper portion (which is why early spring is when sugar maples are tapped--the sap "is rising" and can be collected more easily and les destructively than digging the roots up)
With winter collection, aside from the mostly impossible task of digging frozen soil, trees' reserves that will fuel top growth and recovery are still in the roots. You want to collect at the time when those reserves are being put to use to aid in post collection recovery--ideally, you want to dig when leaf buds are just about to burst open into leaves. That means the tree has committed to growth. Removing roots before the tree begins, or is down the path of this process, weakens the tree.
And BTW, I don't know if you've ever tried to dig frozen soil, but I had a longtime friend who was a gravedigger (someone has to do it). The cemetery team sometimes used small dynamite charges to loosen soil in the winter. A pick and mattock on frozen soil is going to be a bee-otch that won't really pay off.
Also, if you don't have a frost free site to store those collected trees and they freeze, you will have considerable die back on cut roots and the trunk of the tree.
Good luck...