Altering reference voltage of DAC
Works really well. Two small drawbacks:


The gain settings are on a linear scale, so minimum attenuation for an 8-bit MDAC is -48dB. For some applications this may not be enough. In an audio application I augmented it with a 20dB FET-switched gain stage and managed to get pretty close to 1dB per step from 0 to 60, except the bottom 4 steps (0 off, 1 -68dB, 2 -62dB etc).
At high frequencies (well above the audio range) , the internal network is quite complex and so frequency response will not be equally flat at different attenuation settings. I should add that an 8-bit MDAC has the advantage over a 12-bit one; not only is the latter more complex, but in order to meet higher accuracy requirements, it tends to use larger FETs with lower ON-resistance but higher parasitic capacitance so its high frequency performance may be inferior.