I have heard of planting on a piece of 1x10 wood with a few 3/4 inch holes drilled through it. This is like planting on tile, but a few roots will go through and beyond allowing the tree to access more water and nutrients. at potting up, the large roots through the holes are cut off much like sacrifice branches. this will also allow for the nailing technique.
I, however, have noticed that the best way to form nebari is to grow in the ground, in the best soil you can build. In my veggie beds I constantly amend with my own compost, and top dress in the fall with mulched leaves to build leaf mold. in the summer I mulch with a combo of straw and grass clippings (I literally dump the straw on the lawn in thin layers and mow with a bag.) High organic matter in the soil allows the roots to bulk up and acquire nutrients without having to move very far.(Organic matter is by far the most efficient transmitter of plant nutrients.) by planting between and under veggies, my bonsai in these bed develop the best nebari in my garden.