I have a couple of homebrew supplies that use a Variac to control the unregulated DC ahead of a high-current IC/Transistor regulator. To minimize power dissipation in the regulator, the trick is to set the unreg voltage just high enough to keep the regulator from dropping out. It helps to use a regulator circuit with a low drop-out. I provide metering to show both the regulated and unregulated voltage. In another supply, I use a simple comparator to drive a LED; if the LED flashes, the regulator is going into dropout.
Come to think of it, I have a commercial microprocessor-controlled DC supply that uses several relays and a tapped transformer to minimize dissipation upstream of the regulator. I have seen multi-KW rack-mounted supplies that use a servo-motor to set the Variac.
This is certainly not a new idea.