I think recent firmwares would allow Sd-card-less pxe boot. I was wondering the other day if you could then run the whole system from RAM disk only and/or maybe mount volumes via iSCSI from SAN (keeping in mind limitations of RPi 1Gb nic).

Edit: someone has done this already. Of course! :) ( https://tech.xlab.si/blog/pxe-boot-raspberry-pi-iscsi/ )