Risky and would require some testing, but why don't you script it?
Generate your grub.conf via shell script, and have your shell script run on bootup (e.g. put it in rc.local)
Put your scripts in /root/grubscript
Have a wrapper called /root/grubscript/wrapper.sh (which is run by rc.local)
Let's say you have four OS, you would create four grub files
/root/grubscript/grub.conf.1.txt
/root/grubscript/grub.conf.2.txt
/root/grubscript/grub.conf.3.txt
/root/grubscript/grub.conf.4.txt
Your wrapper script will do this (assuming grub.conf is in /boot/grub/grub.conf)
#!/bin/bash
cp -p /root/grubscript/grub.conf.1.txt /boot/grub/grub.conf
mv /root/grubscript/grub.conf.1.txt /root/grubscript/grub.conf.5.txt
mv /root/grubscript/grub.conf.2.txt /root/grubscript/grub.conf.1.txt
mv /root/grubscript/grub.conf.3.txt /root/grubscript/grub.conf.2.txt
mv /root/grubscript/grub.conf.4.txt /root/grubscript/grub.conf.3.txt
mv /root/grubscript/grub.conf.5.txt /root/grubscript/grub.conf.4.txt
You get the idea. Test it manually first by running it from the command line and check that the /boot/grub/grub.conf rotates properly
Finally, for contingency sake, first backup your current grub.conf
cp -p /boot/grub/grub.conf /boot/grub/grub.conf.ORIGINAL
And have an install dvd handy to boot into rescue mode if during your reboots one of your grub files isn't working
Does this help?