Simon,
I wanted to thank you for putting this post together. I also fall into the the camp of planting avocado trees in vain, to see them do good in the spring but as soon as the stress of summer (especially down here) hits they wither up and die. So I also thought to plant some avocado seeds directly on my mound. I planted a bunch last year of all kinds of different varieties with the thought being that there will be enough genetic variation that will produce a rootstock that can withstand our salty water and high temps. I was not even considering the other factors that you point out in the original post (sprouts in the environment it will be living in so it builds a relationship with the soil microbes and chemistry right off the bat, no transplant shock, taproot is allowed to grow unimpeded and undistrubed, etc.). Over this winter I had 3 decent sized seedling (4-6 inches) that had sprouted during the fall and we had a pretty cold snap in December. My yard got to 26 F and stayed there for a few hours for 2 nights. And these seedlings showed absolutely no cold burn or damage at all, I was shocked. Now I have a dozen or so seedlings of various sizes and I will pick 2 or 3 that weathers the summer the best and graft on to those in the fall.
I had given up on growing avocados here, but now I am full of optimism again. Thank you for sharing your process and thought and the success that you have had with it. I love avocados (Reeds are my all-time favorite) and am looking forward to some fall grafting. Thanks again for this post!