Setting a step-down regulator to provide constant drop on the following linear regulator
In the LM317 datasheet you can find and interesting scheme showing how to use two LM317s as a linear regulator with preregulation. The first LM317 maintains the dropout voltage of the second at about 5V. 

If you can find a step-down IC with a sense input which works the same as the LM317, you could do the same trick:



The important thing to note is that the LM317 works by maintaining a fixed reference voltage (~1.25V) between OUT and ADJ terminals. Find a step-down chip that keeps a fixed reference voltage between its sense input and its output and you are done (the sense input must draw negligible current).

In that schematics the calculations are done in this way: the current in R1 is set by the reference voltage across ADJ and OUT:

$$
I_{R1} = \dfrac {V_{ref}} {R_1} 
$$

neglecting the ~40&mu;A absorbed by ADJ \$I_{R1}\$ flows through R2, causing a voltage drop:

$$
V_{R2} = R_2 \cdot I_{R1} = V_{ref} \dfrac {R_2} {R_1} 
$$

Add the reference voltage across R1, i.e. Vref, and you get the dropout voltage of the second regulator:

$$
V_{DO} = V_{ref} + V_{R2} 
= V_{ref} \left(1 + \dfrac {R_2} {R_1} \right)
= 1.25V \left(1 + \dfrac {720 \Omega} {240 \Omega} \right)
= 5V
$$

EDIT (to report more findings)

You can get some ideas from the following sources.

This sites sports a circuit that does what you want: LM317 Adjustable 20V, 1.5A Supply with Simple SMPS Tracking Preregulator (it warns the circuit was only simulated and not built). 

Some application notes on the subject that can be relevant or interesting:


High Efficiency Linear Regulators (Jim Williams - Linear Technology).
Switching Regulator Circuit Collection (John Seago - Linear Technology). Note in particular the pages where switcher preregulators for linear power supplies are shown. For example, this:




The EEVblog video #260 talks about simulating a switching preregulator in the context of Dave Jones &mu;Supply project (there are other videos in that series).