erm pup_ro1 is a aufs layer...yes snapmerge writes directly but inherently you are writing to a aufs layer active in the union. Which also supports the more than one rw layer thought. Nothing goes on outside of the union/aufs.pupmode=13 has only 1 writable layer. "snapmergepuppy" script writes to the save-layer by accessing the directories directly, outside aufs
Perhaps we are at cross definitions here.
mike