It is possible to combine masks using the 3dcalc program. For example, if you have an ROI mask in dataset ROI+orig and a statistical threshold in dataset sub-brick STAT+orig[5] (perhaps from 3dDeconvolve?), you could use 3dcalc as follows:
3dcalc -a ROI+orig -b 'STAT+orig[5]' -prefix ROI_STAT -expr 'step(a)*step(b-4.7)' -datum byte -nscale
The resulting dataset (ROI_STAT+orig) would be 1 where the ROI+orig dataset was nonzero AND the STAT+orig[5] sub-brick was 4.7 or larger, and would be zero otherwise. You could then use this new dataset as the input mask to 3dmaskave (or 3dROIstats, or 3dmaskdump, or ...) to get what you want. When you don't need it anymore, you could delete this temporary mask dataset.
For the advanced user, you can even create "on the fly" datasets using 3dcalc() on the command line. For details on how to do that, see the "CALCULATED DATASETS" section in the output of "afni -help".
bob cox
