I know I had a couple of posts on the subject in 2008/2009 but they seem to have been culled.
This link might be helpfulhttp://pcquest.ciol.com/content/enterprise/2005/105070101.asp
I believe I took a slightly different approach, but cannot now be sure
Here are some notes from 2008 .... cannot say if I followed this scheme or not.
I suspect not, TBH.
what you do is create a folder call it multi-os,
inside that folder you create a folder for each of the distro's you want to boot.
create a boot folder
extract the iso of each distro to it's named folder
extract or copy the necessary boot files into the boot folder (grub etc)
now just gunzip each initrd.gz, one at a time is best, then mount the extracted initrd.gz
this must be done as root, you would need to first make a folder to mount the initrd to
mount -o loop /home/user/Desktop/multi-os/zen/isolinux/initrd /home/user/Desktop/iso
search for BASEIMG (at "line 41" in the example that was noted)
Change that to read
BASEIMG="minime/livecd" (or whatever ISO is in use)
Save the file. Unmount the ISO ... umount /home/user/Desktop/iso
Repeat for each ISO to be used.
then edit menu.lst to show all the distros ..... this is the menu.lst in /multi-os/boot/grub/
An example entry
kernel (cd)/minime/isolinux/vmlinuz livecd=livecd initrd=initrd.gz root=/dev/rd/3 acpi=on vga=788 keyb=us splash=silent fstab=rw,auto
Add an entry for each distro.
Now create an ISO of the lot ....
mkisofs -R -b boot/grub/stage2_eltorito -no-emul -boot-load-size 4 -boot-info-table -o Multi-OS.iso /home/user/Desktop/multi-os/
I cannot say if this scheme can be used today.
All I can say is that it worked in 2008.
Also, I suspect there are easier ways of achieving the desired result these days.
I found a file with some menu.lst entries from such a multi-os ISO --- from about the same year ......
title Puppy Seamonkey
title Boot CentOS-5.2-i386-LiveCD
initrd=(cd)/centos/isolinux/initrd0.img root=/dev/rd/3 rootfstype=iso9660 ro quiet liveimg
title Myah OS
kernel /myahos/vmlinuz vga=791 splash=silent ramdisk_size=6666 root=/dev/ram0 rw autoexec=/sbin/xconf
title Myah OS
kernel (cd)/myahos/vmlinuz vga=791 splash=silent ramdisk_size=6666 root=/dev/ram0 rw autoexec=/sbin/xconf
kernel (cd)/faunos/vmlinuzf vga=791 splash=silent,kdgraphics,theme:faunos quiet=y quiet loglevel=2
Maybe the above will give you some ideas.
It is so much easier to use a USB stick for this these days that I have not played with using a DVD since then.