Commelina is native to moist woodlands - I think you need to be looking at plants tolerant of dry shade. And off the top of my head, I can't think of any that would grow under those extremely minimal soil conditions. And adding soil is not exactly an option. As little as a couple of inches of soil added over the root system/dripline of established trees can effectively smother the roots and eventually kill the tree.

I'd stick with a thin layer of some sort of mulch and call it good. The more textural the mulch - gravel, bark nuggets, wood chips - the less of an issue it is for the tree. If you want color, place a couple of planted containers around the base.