for what its worth, there are people in quebec who have e.g. deshojo and katsura planted in the ground, and at least for the first few years, the roots are pretty shallow. we have a +48" below ground frost line, and in november/december we normally get many spurts of several consecutive days of -30C (-22F) before any snow has fallen (i.e. before the snow insulated the roots). i don't know what the temperature of the ground is when the air is -30, but i can tell you that the loose soil in my vegetable garden is a 36" deep frozen block of ice from december to march
i would add that bloodgood (and sentinel? or something like that) is viewed as the most winter hardy of 'japanese maples' in quebec's box stores - its often the only variety they sell
been speaking to people more and more about growing maples in the ground in quebec, and i keep learning that jeff goldblum was right: life finds a way