How to scan an entire empty ZFS raid-z array for bad sectors (blocks)?

ZFS does not have methods to scan unassigned blocks, because it serves no direct purpose:

• If you have deleted data and want to recover it, you can access the low-level features with zdb. There are also some scripts on the web that try to scrape for usable data on a destroyed pool, although the easier way would be to have backups.
• If you assume that your drive is faulty (high read/write error counts or weird noises), you generally replace it right away, because the data is more valuable than a single spare drive. If you really don't have much budget, you could scan/verify the disk with low-level tools from the drive manufacturer to detect bad sectors and mark them unusable. Of course, this might result in a smaller disk which may be too small for your existing pool. Also, while scanning without a replacement your pool may be vulnerable depending on layout (mirror, raidz1).
• If you do not care about the unassigned blocks, there is no need to examine if they have changed. One could argue that hidden errors could accumulate on sparsely used disks, but this use case is not the majority - after all, if you never fill your pools, you could have bought smaller disks (or more of them).