Here in Texas in many places the bedrock is about 6" under the soil so to plant a tree people just pile up soil, in essence a raised bed, and plant. Been doing it for 200 years and some of the trees are that old. If your raised bed does not have a bottom to it you should be fine.
I've read a few sites that say that planting the rootball a little above the ground is helpful in clay soil. (Which makes sense to me - about a year ago I read that if an older tree starts growing big roots along the surface of the ground then chances are, the roots are starved for oxygen.)
University of Minnesota Extension said:
If the soil is particularly heavy, both trees and shrubs benefit from being planted so the top of the rootball is slightly higher than the adjacent ground. The optimum amount to raise the rootball depends on its depth. Add soil to the bottom of the planting hole so the plant's rootball, when positioned, will be raised above the adjacent soil level by one inch for every eight inches of rootball depth. http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/trees-and-shrubs-for-clay-soil/