It seems that there should be a way to do this with sdc-cnapi. I've not had luck in getting any updates to flow from sdc-cnapi to booter, so I tried another approach.
root@headnode# sdc-login dhcpd
...
root@dhcpd0 ~ # cd /tftpboot/00505634604c
root@dhcpd0 /tftpboot/00505634604c # mkdir etc
root@dhcpd0 /tftpboot/00505634604c # cd etc
root@dhcpd0 /tftpboot/00505634604c/etc # echo "this is a test file" > testfile
root@dhcpd0 /tftpboot/00505634604c/etc # digest -a sha1 testfile > testfile.hash
Restart the dhcpd service to have it rewrite the boot config files.
[root@dhcpd0 /tftpboot]# svcadm restart dhcpd
If you don't restart it, eventually some magic will cause /tftpboot/boot.ipxe.0100505634604C and /tftpboot/menu.lst.0100505634604C to be updated.
The boot.ipxe file:
#!ipxe
kernel tftp://${next-server}/os/20190619T233708Z/platform/i86pc/kernel/amd64/unix -B hostname=headnode,rabbitmq=guest:guest:10.99.99.20:5672,smt_enabled=true,rabbitmq_dns=guest:guest:rabbitmq.madtown.local:5672,admin_nic=00:50:56:34:60:4c,external_nic=00:50:56:3d:a7:95,console=ttya,ttya-mode="115200,8,n,1,-"
module tftp://${next-server}/os/20190619T233708Z/platform/i86pc/amd64/boot_archive type=rootfs name=ramdisk
module tftp://${next-server}/os/20190619T233708Z/platform/i86pc/amd64/boot_archive.hash type=hash name=ramdisk
module tftp://${next-server}/bootfs/00505634604c/networking.json type=file name=networking.json
module tftp://${next-server}/bootfs/00505634604c/networking.json.hash type=hash name=networking.json
module tftp://${next-server}/bootfs/00505634604c/etc/testfile type=file name=etc/testfile
module tftp://${next-server}/bootfs/00505634604c/etc/testfile.hash type=hash name=etc/testfile
boot
[root@f552537d-de3a-46b4-a9c4-9cccf3def86c (madtown:dhcpd0) /tftpboot]# ls
boot.ipxe.0100505634604C cache menu.lst.0100505634604C poolcache snponly.efi
bootfs joybadger.xpm.gz os pxegrub undionly.kpxe
The menu.lst file:
default 0
timeout 5
min_mem64 1024
variable os_console ttya
serial --unit=0 --speed=115200 --word=8 --parity=no --stop=1
terminal composite
color cyan/blue white/blue
title Live 64-bit
kernel$ /os/20190619T233708Z/platform/i86pc/kernel/amd64/unix -B hostname=headnode,rabbitmq=guest:guest:10.99.99.20:5672,smt_enabled=true,rabbitmq_dns=guest:guest:rabbitmq.madtown.local:5672,admin_nic=00:50:56:34:60:4c,external_nic=00:50:56:3d:a7:95,console=${os_console},${os_console}-mode="115200,8,n,1,-"
module$ /os/20190619T233708Z/platform/i86pc/amd64/boot_archive type=rootfs name=ramdisk
module$ /os/20190619T233708Z/platform/i86pc/amd64/boot_archive.hash type=hash name=ramdisk
module$ /bootfs/00505634604c/networking.json type=file name=networking.json
module$ /bootfs/00505634604c/networking.json.hash type=hash name=networking.json
module$ /bootfs/00505634604c/etc/testfile type=file name=etc/testfile
module$ /bootfs/00505634604c/etc/testfile.hash type=hash name=etc/testfile
title Live 64-bit +kmdb
kernel$ /os/20190619T233708Z/platform/i86pc/kernel/amd64/unix -d -k -B hostname=headnode,rabbitmq=guest:guest:10.99.99.20:5672,smt_enabled=true,rabbitmq_dns=guest:guest:rabbitmq.madtown.local:5672,admin_nic=00:50:56:34:60:4c,external_nic=00:50:56:3d:a7:95,console=${os_console},${os_console}-mode="115200,8,n,1,-"
module$ /os/20190619T233708Z/platform/i86pc/amd64/boot_archive type=rootfs name=ramdisk
module$ /os/20190619T233708Z/platform/i86pc/amd64/boot_archive.hash type=hash name=ramdisk
module$ /bootfs/00505634604c/networking.json type=file name=networking.json
module$ /bootfs/00505634604c/networking.json.hash type=hash name=networking.json
module$ /bootfs/00505634604c/etc/testfile type=file name=etc/testfile
module$ /bootfs/00505634604c/etc/testfile.hash type=hash name=etc/testfile
title Live 64-bit Rescue (no importing zpool)
kernel$ /os/20190619T233708Z/platform/i86pc/kernel/amd64/unix -B hostname=headnode,rabbitmq=guest:guest:10.99.99.20:5672,smt_enabled=true,rabbitmq_dns=guest:guest:rabbitmq.madtown.local:5672,admin_nic=00:50:56:34:60:4c,external_nic=00:50:56:3d:a7:95,noimport=true,console=${os_console},${os_console}-mode="115200,8,n,1,-"
module$ /os/20190619T233708Z/platform/i86pc/amd64/boot_archive type=rootfs name=ramdisk