Hybrid power supply output voltage
You could try a circuit like the one below.
R1 provides bias for the zener, while R2 and C form an RC filter to reduce the unwanted ripple from the switched power supply (if you have no filtering, you defeat the purpose of using a linear regulator because the LM338's output voltage would carry all the ripple from the switched regulator).
The LM338 output voltage should follow its input voltage minus ~3.9V, what provides an appropriate dropout voltage for this regulator. The output voltage could be controlled by a potentiometer connected to the feedback line of the LM22679, as you suggested.

Disclaimer:


I haven't tested or simulated this circuit
the choice of values for the discrete components may be a little tricky  


Other comments:


I'm not sure how clean you need your output to be. Keep in mind that you can reduce the ripple of the switched power supply by simply increasing the inductor and capacitor values. Depending on the load, you could also have a simple RC filter in front of it to further reduce the ripple from the switched power supply







simulate this circuit &ndash; Schematic created using CircuitLab