Measure CMRR of amplifier in a single HSPICE simulation?
I'm not very familiar with HSPICE (mainly LTSpice), but I think this can be done with .MEAS and .STEP commands, which I believe are common to both environments. 

Basically, design a step command such that one run will configure the sources so as to measure differential performance, and the second run will configure them as common-mode. 

Then, use a measure command to measure the output amplitudes of each run, and a second measure command to find the ratio of these two.