We are trying to use another DAC to adjust the voltage across RSET to allow for adjusting the Full Scale Current Output. Has anyone done this before? Is this possible?
It is not commonly done but should be possible. It needs to be a current mode DAC, and the direction of current from RSET to ground. You also need to be careful about output capacitance of your adjustment DAC as it may destablize the reference loop (figure. 78 in datasheet). You will likely suffer some noise degradation.
Alternatively you could use external reference for REFIO, or use a digital potentiometer.