Is there a simpler way to create a variable voltage drop than a differential amplifier?
The TL431 you mention can be used to create a variable positive or negative voltage from 2.5 to 37V  up to 100mA limited of I*V=Pd drop rating of the package and temperature rise.

To make it a zener for negative voltages the series R goes on the cathode instead and anode is your fixed reference point while cathode is the output voltage biased by a more negative input series R.

Using a Triac primary control with optoisolation  for pre-regulating DC output into a high power LDO was a basic method used by Lambda in the 70's in lab supplies.  Their front end was much more sophisticated than I am suggesting in order to regulate heat, surge current and minimum Dc with ripple for the linear regulato. 

It worked with better pre-regulated voltage to prevent needed higher no-load voltage rated capacitors and had oodles of LC filtering,  but it offered a smaller package size per 100W than anyone else.  Still  nowhere near as good as a SMPS.