Texas instruments has an app note and video explaining how to make a programmable output power supply using a typical LDO voltage regulator and a DAC. This is the technique we used for the Bus Pirate Ultra power supply to get 0.8 to 5volts output, and it works a treat!
Consider the currents going in and out of the VFB node shown in Figure 3, which is connected to the ADJ pin of the LDO. Almost no current flows in or out the device through the ADJ pin (on the order of 0.01µA). As I previously mentioned, the output voltage of the LDO is always produced such that the voltage at the ADJ pin – and therefore the VFB node – is equal to the LDO’s internal reference voltage. Thus, the current through R2 is constant. It follows that any sourcing or sinking of current by the DAC through R3 is reflected as a proportional voltage increase or decrease at VOUT to compensate for the changing current that must flow through R1.