I'm putting in several keyholes in my yard, mainly underneath crepe myrtles where plants can get some shade (in zone 8b) to have a chance to grow as our Texas heat can be brutal. As far as the tree roots, I'll be putting down landscape fabric underneath to keep them in check. In addition to lots of cardboard, paper, and compost, have heard that when adding tree limbs that small logs only up to an 1 inch or so in diameter should be used as bigger logs with use up the nutrients longer as per Dr. Deb Tolman - www.debtolman.com.
Have also seen someone using big logs especially for transplanted plants where a log sits upright underneath the soil and serves as a sort of base to support roots. The plants are in a net pot. Here' someone that's been experimenting with - http://lowcostvegetablegarden.blogspot.com/2012/07/shock-wilt-free-transplanting.html