In my search, I only came across a probing question as to if there were plans for iSCSI back in 2008 but not much else. Is there an option for initiating iSCSI through pfSense?
I'd love to use it for a "local" ZFS fileshare for holding ZFS replication of the boot drive. From looking around, I see "istgt" is frequently mentioned in FreeBSD as a package that can initiate iSCSI connections. Did a quick pkg search and didn't see it in the pfSense package repository. A driveless pfSense configuration would also be kind of neat although I wouldn't want to run without a primary real localized instance for my router.
I don't know if this would work but I can say it is ill advised from a security standpoint..
You don't want your internet facing firewall to do anything but routing and network tasks.
iSCSI file serving is something you want to do behind pfSense not on top of it.