According to the article the ONLY way to expand ZFS after the fact is to "replace every hard drive in the VDEV, one by one, with a higher capacity hard drive." If you have some better work-around that gives you both, you need to either explain it or link to an article that does.

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.