You can't drive the clusterize plugin directly yet, but you can set the threshold from a plugout with the SET_THRESHNEW plugout command. If you do the threshold and clustering through 3dclust or 3dmerge, you can use the plugout to view the resulting dataset, but you may need to insert a "RESCAN_THIS" plugout command. Also take a look at this previous thread:
http://afni.nimh.nih.gov/afni/community/board/read.php?1,124376,126084#msg-126084