I was able to use ndmpcopy to dump /home from F520-old to F520-new in about 6 hours. (Not too bad.) I'm not quite sure this makes sense, but can .snapshot be ndmpcopied too?
Naively trying ndmpcopy oldfiler:/.snapshot newfiler:/vol/vol0/.snapshot generated a
subtree path must refer to a specific snapshot name in ".snapshot".
error message. But trying weekly.1, specifically, then generated a
newfiler: /vol/vol0/.snapshot/weekly.1 - cannot create directory: Read-only file system
error message (after spending 36 minutes mapping files and directories).