I was wondering what options were used to create the stock Solaris ISO images. I found this in Creating a Customized Boot CD/DVD for the Solaris™ Operating System for x86 Platforms
# mkisofs -r -no-emul-boot \ -b .bootimage \ -c .catalog \ -G genboot \ -o composite.iso \ -sunx86-boot /var/tmp/my.miniroot \ /tmp/isodir/
But I know this is pre-grub. Hmm, I'm not generating my own miniroot, which is a difference.
The -sunx86-boot does generate the El-Torito magic needed. But I'm guessing that grub is replacing the miniroot.