This was frustrating:
# zpool import nas2
Assertion failed: rn->rn_nozpool == B_FALSE, file ../common/libzfs_import.c, line 1086, function zpool_open_func
Discovered that I needed to tell zpool where to look for the devices:
# zpool import -d /devices/scsi_vhci nas2
There was a time when I would have known that already, if we still used Solaris in any way at work. I wish the error message was more descriptive. Considering asking about it on the openindiana or illumos mailing lists.