The frozen rootball method sounds plausable. In my life before being a treeguy, I was into Bonsai for several years. The best looking bonsai specimens-to-be had large trunks proportionate to the size of the crown. Large trunks will fortell of a large root system and to become a bonsai, that plant must be transplanted into a small pot. This means fairly drastic root pruning.
Summer specimens were easy to find, junipers at any garden center or nursery. I favored K-Mart as they were the cheapest, but substantial root pruning during the peak growing season often led to failure, about half. I went to bigger pots and less root pruning with Summer stock.
In the Winter, garden centers (at retail outlets anyway) often don't stock plants because nobody is buying. I would have to source my stock from the 'wild', quick and stealthy under guise of night. These plants would often get roughed up pretty bad during the often hurried procedure and when we got home (me and the plant) I'd further prune the roots and then wire it into the pot. Rarely did these die and in the Spring they'd wake up and do what they do, only in a new home.
Clearly, bonsai is at the opposite end of the size spectrum as to what this thread is about, but similarly, the plants being used were somewhat mature, established (though small) trees. The big ones you're asking about are way, beyond sapling, somewhat mature and fully established at the time of the move. Both are examples of transplanting trees. The biology is very much the same.