My thinking was that expanding your underlying md RAID would be the same as replacing the initial disk ZFS sees with a bigger one, thus enabling easier expansion at the md level and presenting a "bigger disk" to the zfs vdev.

I haven't seen it done, it's just a theory, hence why I asked. I'm just not sure if zfs needs to see actual disks, or if it can work on top of any block device, like an md RAID.