You can get motorized dampers that you install where the branches come off the main trunk.

My understanding is it's not great to shut down vents all the way since it increases the static pressure in the branch and the air is just going to try to find ways out. You also have to be careful with closing down too many registers as it can cause too much pressure on the furnace. Some furnaces will shut down in this case.

Right, with the smart dampers I dream of they would just be placed as register covers throughout the house, and would communicate with each other and the furnace to regulate house temp. So they would never try to close many at once, it's more like the cold room opens wide and the rest close a little bit.

In my fantasies the central controller can learn how much to open each vent until each room heats up at about the same rate. Presumably that would end up with one or more vents fully open, and the more insulated rooms progressively more closed.