There's certainly a reasonable way to implement this, which would involve some sort of bitmap of written-to and not-written-to blocks, but, as I recall, Linux RAID doesn't keep any metadata on disk, so there's not any way to record this metadata. (Correct me if I'm wrong about that.) Not that all (or even any, necessarily) on-disk metadata RAID systems implement this. Recurring storage and the need to journal that data probably outweighs the cost of a one-time setup.