Is there an existing type of voltage regulator that holds a desired/specified voltage below the Vin?
You merely have to connect a negative voltage regulator of the desired "drop voltage" to the positive rail. Then, connect your load between the output of  this negative regulator and the "ground" or DC Common or negative rail, of your voltage source. 

Look at negative regulator ICs such as 79M05 (-5 VDC), 79M12 (-12 VDC), etc.  for fixed voltage drops. There are also adjustable negative voltage regulators which can make almost any (reasonable) drop you need.  

The 79Mxx series will work up to a certain supply voltage. Eg. from Vin = 6 up to maybe 35 volts or so. For higher Vin voltages you will have to identify a more  appropriate family. There is a wide variety of  both positive and negative IC voltage regulators in the marketplace. Check DigiKey and Mouser.