I have seen full sized trees being trained or supported with structures put in place to keep their growth where intended. I had never thought about it being applied to bonsai though. However, while trimming is a key part of shaping a bonsai, there are more direct approaches like wiring along branches to keep them going as intended. A close look at some of the exhibits at the Pacific Bonsai Museum showed how this was done. The clever bit was that these additions were not so conspicuous and didn’t ruin the appearance of the tree.