OK, I found these instructions buried in another post:
lundman wrote:zpool export POOLNAME
zpool import -d /dev POOLNAME
zpool online -e POOLNAME /dev/diskX
<confirm expansion>
zpool export POOLNAME
zpool import -d /var/run/dsk/by-id POOLNAME # was that the path?
lundman, ilovezfs, someone else with wiki edit privs: can you PLEASE add this to the documentation section? This is clearly OS X-specific.