Basic outline:
1. Boot of an alternate media, like a live cd or usb stick.
2. import the root pool. to do this you need to specify an alternate mount point.
e.g.:
# zpool import -R /oldbe rpool (this specifies you mount in /oldbe)
3. access the ZFS BE from the /oldbe directory (or wherever you have mounted). you can now perform any repairs needed.
4. after repairs complete reboot the system ( # init 6)