Phillip, I like your idea of using the rocks as heat sinks...have you gone out on cold nights to compare the air temperature inside a cluster of rocks with the air outside the cluster. If it making a difference, then as the tree grows up, you could pile rocks on top of rocks and have a rock greenhouse of sorts. That would also have the side effect of keeping sun off the delicate avocado bark. You'd just have to make sure the rock piles were stable, and that your area is earthquake proof.
You're on the right track with the Duke 7, but it's true that Duke 7 has been used as a rootstock for many years (less so now I believe). The reason is that it has root rot resistance and is also graft compatible with commercial varieties. They don't seem to be sold as fruit producing trees, at least I have never seen them mentioned in lists of cold hardy trees.
There are some large Duke trees still growing in the Oroville, California area, and there are still some Duke trees remaining from part of an orchard in the Bangor, California area. Apparently they still sell the fruit at local markets in the Oct/Nov period.
About 1 1/2 years ago I obtained some "Duke" seeds from two sources in the Sacramento, California area. One source was from someone who thought he had gotten them from one of the original Duke trees, and the other source was from another local area person who had visited a large old Duke tree next to the old railroad station at Oroville and taken scionwood. He had someone in his area graft this Duke scionwood onto rootstock grown from local (Waterford, Calif.) area cold hardy seeds. One of his grafted Duke's grew a huge amount in two years and produced fruit...and he sent me 3 seeds from that fruit. So I'm trying to grow Duke seedlings, from seeds from these two sources. My climate isn't as cold as yours, but it's cold enough (24-28F each winter) that Hass and Fuerte are very marginal. I have a grafted Fuerte, and some Fuerte seedlings, growing in contained outdoors and they've handled 28F(-2C) without any problem, but it doesn't stay that cold for many hours here. Neither Hass or Fuerte are considered cold hardy varieties (and the list in one of the messages above is incorrect listing Fuerte above Mexicola as it's not even close in cold hardiness to Mexicola).
My Duke seedlings from the 2nd source died on me, probably from not watering them correctly in the hot summer weather here. I have about 5 of the other source seedlings still in small containers and cups under grow lights indoors.
I hope this information is interesting to you, and although it would be surprising if your Duke 7 had a Duke grafted onto it, even if the graft is killed by cold weather it's worth it to let the rootstock keep growing. I don't know what quality fruit it would produce, but it has the right genes for cold weather.
I'm attaching a pdf file from a 1963 California Avocado Society yearbook, some pages about the Duke avocado and it's origins.
Also, here's a link to a small northern california nursery that tests, grows and sells cold hardy varieties. I've been wanting to buy a Bonnie Doon from them but they are a small nursery and often sold out. http://www.epicenteravocados.com
. It's a good source of information.