I garden with clay soil, and have lost a tree before to the 'bowl effect'. Planting trees directly into the native soil has worked (best with mulch added over that soil!), but the flowering cherries I did this with have grown slowly (or so it seems) over the years. (That could be because of Japanese Beetle damage every summer, or higher levels of salt in the soil near the road. An application of Gypsum should help with the latter; I'm trying that this year.) Anymore what I do, is build raised beds out of a desirable mix of 100% bark & mushroom soil (or other compost soil) that is locally available - right on top of cardboard to control weeds from the existing meadow lawn - and plant new things as deeply as I can in the raised beds. This creates a layered effect that mimics the natural layers of mulch in a forest, and keeps the top 8-10" of soil very workable for my purposes as well.
I do wonder, Daisyl: why do you think it is important to keep soil away from the tree trunk? I hear about this regularly, but have never really seen a reason why to do this with young trees being newly planted... unless it is to make sure the grafted cultivar -can't- root into the surrounding soil. (Unless the specific rootstock used is required for a necessary tree attribute, such as a dwarfed habit, why wouldn't you want it to?!) Just wondering.
Anyway, I figure this practice allows the roots to establish in the loose soil that is more prone to stay drained even in a wet spring, then naturally overflow into the clay soil beneath as soon as it's ready. Part of the reason I like to plant things deep (with the exception of various perennials that shouldn't be) is to have the roots down where the soil moisture stays consistent (most of these are planted beyond the sprinkler zone), and the tree has a better chance of getting successfully established. (I would NOT plant them deep in the clay, except in special cases. Only deep in the mulch/soil raised bed.) It also gives any small tree or shrub a better anchor against deer who should find it appealing enough to tug at while it's still so small. Many plants will eventually root out from the buried stem(s), and develop a better-established base because of it. If it works to do so, pruning a non-grafted woody plant low to the ground while dormant, seems to be a good way to protect it from deer predation over its first winter - even as it encourages a better root to top ratio that will push out a stronger set of new growth in the spring (and hopefully be able to better hold its own against critters).
All in all: just a few ideas to play with, if you think they'd work for you.