Is there a way to make leafy green vegetables absorb salt throughout their tissue?
When the greens are cooked, the cells will rupture and more easily take on outside fluids (and salt), so salting them while cooking will get the salt into the greens. You could also brine or rub the leaves with salt, this is frequently done with cabbage before pickling. The effectiveness of this method will vary depending on the greens though, some leaves like kale have a more waterproof layer over the outside of the leaf, so just rubbing them with salt may not work. I have seen people "massage" the leaves with salt to tenderize them and break down some of the fibers, or blanch them in salted water.