Hi Paul,
Instead of using 3dclust, start with "3dmerge -1clust_order"
(maybe see example #5 in '3dmerge -help'). That will give
you an ordered set of masks, from 1 to N, where a voxel
with a 1 was in the largest cluster, 2 means next largest,
etc. Call this dataset clust_mask.
Then use 3dROIstats with '-mask clust_mask+orig'. That will
give you results over each of the cluster ROIs.
Hopefully, that would be it.
- rick