The same question was posed to me about the global signal regression vs. CompCor.  I like your response, but I suspect it may not satisfy them as it has become hardened in some that global signal regression is always required.  One thing we did was to collect the global signal intensity values from our groups during CONN preprocessing (output in MATLAB) and then compared these values to see if there was a significant difference between groups and over time.  This helps buttress that there aren't any statistically significant differences between groups, but I suspect even this may not placate some reviewers.

One question I have is what is the "Effect of Condition" regressor controlling for during the denoising process?  Is this a condition specific global signal regression variable?  

Jeff