I buried a 3" pot of wild blackberry a friend of mine sent me last year. Being that it was only 3" it wasn't that deep in the ground, but luckily we had that gigantic snow storm last year and I think that is what insulated it and it survived!
I dont think you *can* actually keep the plants from freezing in zones 6 and below..
and once they freeze, they can stay frozen for months..thats really why it doesnt work..
Scot