1- In the boot partition :
Create a frugal base slitaz initramfs including http://hg.slitaz.org/slitaz-arm/raw-file/tip/rpi/piboot
Create one subdirectory per OS and according config.txt, kernel, cmdline.txt and initramfs if needed.
(Move config.txt and cmdline.txt to the raspbian subdirectory)
Create a config.txt in root directory to start the frugal slitaz.
Create a menu.txt in root directory like:
title=Boot menu
subtitle=Choose the OS to start
default=slitaz
timeout=30
kmap=us
Create a cmdline.txt in root directory like
root=/dev/mmcblk0p2 rootwait kmap=us rdinit=/sbin/piboot quiet
The file slitaz/cmdline.txt will look like
root=/dev/mmcblk0p2 subroot=var/slitaz rootwait kmap=us quiet
see http://doc.slitaz.org/en:guides:uncommoninst#subdirectory-install-in-a-posix-filesystem
2- In the raspbian partition, install slitaz in var/slitaz
3- Optional: you can have a full slitaz frugal boot entry (even with 256Mb RAM), a web boot entry ...