Here is the Grub4Dos menu entry:
find --set-root /boot/grub/core.img
That should do it. You'll need Grub2's system files, of course. I run several USB drives on which I installed Grub2 on as per http://www.pendrivelinux.com/install-grub2-on-usb-from-ubuntu-linux/
Note: I did not grub-install on the USB I'm multibooting, I used grub-install on another USB drive and copied /boot/grub/* over to my super multiboot USB.
You can use my Grub2 installation if you don't want to bother making your own. I included a sample grub.cfg file. This should go in your USB drive's root directory, so that you get something like /boot/grub/core.img
Grub2 USB Installation (MediaFire)