AHA, now I understand...
You cannot dedupe from Snapshot to Active File System.
The data in the Active Files System can be deduplicated (if the content allows it, and you are right, the kind of file you are testing with does not work). The SnapShots are a READ-ONLY copy of the AFS-inode (incl. pointers) and therefore cannot be deduped separately.
All you can get deduped is the data in the AFS, then snapshot the volume and save space in the snapshot as well, beacuse the blocks are already multipointered by the AFS-dedupe process.
I hope this answers your question,
Peter