We are fortunate here in that, the areas that have clay only have a 2-3 foot layer, so I dig down to the gravel an backfill with ammendments...
If that isn't possible, I would imagine that a convex base of the hole under the rootball, may indeed work...
Your idea of capping the top of the ground so the water runs away is also a good idea,,, but I always like to be able to dump water into the soil immediately around the trunk...
If you're able to keep the roots wet right at the base of the tree, it won't matter how dry it gets at the drip line... so just estimate how much water it would take to soak up the root ball once a week and use only that amount after your air bubble soaking at planting time...
You might have to monitor plantings for the first couple of weeks personally, just to figure out the correct watering schedule for the h.o. on a case by case basis...
I charge for 2 or 3 visits in the cost of planting just becuz I hate to yell at the client for killing or damaging a plant...
