I am not sure what "robust intensity range" means, but if you would like to scale each time series by its mean, then you can do this (extracted from afni_proc.py output):

# ================================= scale ==================================
# scale each voxel time series to have a mean of 100
# (be sure no negatives creep in)
# (subject to a range of [0,200])
foreach run ( $runs )
3dTstat -prefix rm.mean_r$run pb03.$subj.r$run.blur+tlrc
3dcalc -a pb03.$subj.r$run.blur+tlrc -b rm.mean_r$run+tlrc \
-expr 'min(200, a/b*100)*step(a)*step(b)' \
-prefix pb04.$subj.r$run.scale
end

These steps are described in glorious detail in the tutorial here:

[afni.nimh.nih.gov]

See the t14_scale.txt page there for more info.