Why not just use lilo to boot Sabayon? Though I suppose you
could use lilo to chainload grub and then have grub load Sabayon. It just seems to me like extra work.
This should load sabayon if you're not using an initrd. if you do need an initrd, if you need an initrd be sure to use the /boot/README.initrd in the slack boot dir.
Code:
image = /boot/vmlinuz
root = /dev/sdb1
label = Sabayon
read-only
that should search /dev/sdb1 for /boot/vmlinuz and boot it.