Thanks Rick. I've tried to do a little bit more research and have discovered actually quite a few articles about resting-state connectivity in which they plot t-statistic maps instead of mean z-transformed correlations, e.g.
- Greicius et al. 2003 Functional connectivity in the resting brain: a network analysis of the default mode hypothesis; or
- Schwarz et al. 2013 Anti-correlated cortical networks of intrinsic connectivity in the rat brain.

However, I haven't been able to find any methodological discussion of this issue. When plotting the t-statistic map, we basically plot how significantly different from zero the correlation with the seed region time course is. In a way, this captures the notion of a connection between the seed and target regions (or voxels), although I still find the mean z-score more intuitive.

In any case, is there any reason to use one over the other?

-John