I want to control Q1 in its saturation region so that it behaves like a resistor that decreases in resistance as VDAC increases

The SMPS is a closed loop system which needs to be properly compensated for stability. What you want to do is similar to inserting a potentiometer in the feedback, and creates a variable feedback factor, and thus the loop will need to be compensated properly for all settings. It might work... or not...

It is much easier to inject a current into the feedback node. This works by addition instead of your proposed multiplication, it does not modify the feedback factor or the stability, or the loop gain, etc.

In other words, just do what Andy says (resistor from DAC output to FB node). This answer is only to explain about the loop gain and compensation gotcha.

I want to control Q1 in its saturation region so that it behaves like a resistor that decreases in resistance as VDAC increases

The SMPS is a closed loop system which needs to be properly compensated for stability. What you want to do is similar to inserting a potentiometer in the feedback, and creates a variable feedback factor, and thus the loop will need to be compensated properly for all settings. It might work... or not...

It is much easier to inject a current into the feedback node. This current can be positive or negative depending on which way you want to adjust the voltage. This works by addition (or substraction) instead of your proposed multiplication, it does not modify the feedback factor or the stability, or the loop gain, etc.

In other words, just do what Andy says (resistor from DAC output to FB node). This answer is only to explain about the loop gain and compensation gotcha.