Daisuke Aoyama (Google translation) has created a kernel module to glue an iSCSI disk to a boot drive. While the driver was intended for use with iBFT, you can also use it with gpxeboot. He has even made FreeBSD install ISOs with iSCSI support available. The server has limited bandwidth, so start the download well before you want to use it. (I have previously looked at using iSCSI disks on FreeBSD.)
Thank you, Aoyama-san!