Time Machine, ZFS and deduplication
Deduplication on ZFS is block-level, so it doesn't depend on the concept of files. The only way the deduplication can be defeated is if the offset of a file within the sparsebundle is not always the same modulo the block size. Since ZFS can use variable block sizes that are larger than the block size of the HFS+ file system inside the sparsebundle, correct alignment isn't guaranteed, but neither is it guaranteed that deduplication will fail. 

If you're worrying about space, you may also want to enable compression on your ZFS pool. It imposes extra CPU overhead, but can actually increase effective disk throughput.