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)