There was a cute trick at the last meeting of our local society. One guy put tin foil around the base of the tree, reflecting sun up to the lower branches of the tree. It caused increased growth of leaves on the lower part of the tree. It also reflects heat away from the pot, which can't hurt the tree.
Interesting, there's a large scale Bonsai producer that uses tin foil around the sphagnum moss on airlayering, I don't think it's the best, but it works, and it's fast, since it's a large scale field and do that by the thousands, it fits it's needs.
After some discussion we decided that what the presenter did was put tin foil on the stand around the pots, not covering the soil. An opinion was expressed that a flat white sheet of something would zdo better, diffusing the light more.
This seems that it would heat up the tree without benefit of reducing sunlight on the soil. I think the presenter has a lot of shade around the trees, and mine being in full sun might not benefit.