Looks like multipathing via the generic scsi_vhci may be possible. Thus far:
Add this to /kernel/drv/scsi_vhci.conf:
device-type-scsi-options-list = "NEXSAN SATABeast", "symmetric-option";
symmetric-option = 0x1000000;
Currently cfgadm -c configure c4 is failing, complaining about being unable to create device nodes.
Having said that, the single-pathed zpool create Just Works.
Work progresses…