Perhaps reserve a partition on the spinning array and performing incremental differential at defined times and usage-levels so the loss of the array isn't bad ... but a key aspect would depend on preserving enough room on the metadata vDev to basically make a flat image (like a sparsebundle ... thus, when those backups occur it involves tracking what changed and updating it as a large file ... in the way that an image file contains many small files but is transferred at full-speed because it's doing a block-level transfer. (sorry, I'm tired right now) ... anyway, this'd keep the potential losses low without requiring a bunch of expensive drives as an array but rather a mirrored set.
thanks
thanks