I've not seen this anywhere on t'interweb so this is what I have done so far which seems to work.

Only for arch x86 & amd64.

Tools & Files.

Syslinux Version 3.86 at the time of writing this.

Grub (more commonly used)

Gentoo (amd64, x86) You'll need the iso files.
Stage3 and portage archives are optional, but handy to get the install started.

Prep.

Create a folder somewhere.  This will be the root of the DVD

ISOLINUX METHOD Unpack the syslinux archive. cd in the syslinux directory and run "make". cd back into the root of the DVD directory. mkdir -p boot/isolinux cp syslinux/com32/menu/vesamenu.c32 boot/isolinux (needed for boot menu) cp syslinux/core/isolinux.bin boot/isolinux (needed to boot) create isolinux.cfg in boot/isolinux (needed for boot menu) put a 640x480 png image in boot/isolinux (optional background

GRUB METHOD mkdir -p boot/grub cp /boot/grub/stage2.eltorito boot/grub (needed boot image) cp /boot/grub/splash.xpm.gz boot/grub (optional background picture) create menu.lst in boot/grub (needed for boot menu)

put the stage and portage archives in the root folder of your CD. (optional)

touch livecd in the root folder of your CD. (needed for livecd to load)

mount each iso and copy the gentoo.efimg and image.squashfs files into the root folder of your CD.  (contains the gentoo environment)
I found here that you will need to rename the files.  I just replaced "gentoo" & "image" with the arch.
ie x86.efimg & amd64.efimg etc
copy the gentoo.efimg.mountPoint directory into the root folder of your CD.  (contains the kernel and initrd)
Again replacing the "gentoo" with the arch.

Create the ISO.

cd outside of the root of your CD folder and run

ISOLINUX METHOD mkisofs -N -V gentoo -b boot/isolinux/isolinux.bin -d -iso-level 4 -no-emul-boot -boot-load-size 4 -boot-info-table -o Gentoo.iso gentoo-multiboot (the isolinux.bin is in your CD root.  the gentoo-multiboot is the name of the root of your CD

GRUB METHOD mkisofs -R -b boot/grub/stage2_eltorito -no-emul-boot -boot-load-size 4 -boot-info-table -o Gentoo.iso gentoo-multiboot

Testing.

I have qemu installed but any VM will do.

qemu -k en-gb -enable-kvm -m 1024 -cpu qemu32 -cdrom gentoo.iso -boot d

note: Add "-vnc :0.0" if qemu is on a remote server.  The just vnc to it :)