There has be to a way. I have tried so many variations using ipxe and regular pxe.
I have managed to get to the menu and boot, but then it eventually tries to mount the iso, or the img, and it panics and dumps because it cannot. It is late,
I have tried variations, shots in the dark, and hell, some pure guessing, what I have below is where I am at now and giving up for the night
set os_root os-images/freebsd/opnsense/22.7/
kernel --name opnsense http://${server_ip}/firmware/memdisk
#initrd --name initrd http://${server_ip}/${os_root}/OPNsense-22.7-OpenSSL-dvd-amd64.iso
initrd --name initrd http://${server_ip}/${os_root}/OPNsense-22.7-OpenSSL-serial-amd64.img
#imgargs opnsense append harddisk raw netboot=nfs nfsroot=${server_ip}:${root_path}/${os_root}
imgargs opnsense initrd harddisk raw
Really.. no one.. no one has ever setup opnsense to pxe boot....
FreeBSD likes to netboot via NFS - maybe my blog article is helpful. Never tried OPNsense, though.
https://punkt.de/de/blog/2017/automatisierte-installation-von-servern-mit-freebsd-und-zfs.html