There is no need to let grub be in MBR. Just install it to (hd0,6) then dd out the first 512 byte of it and follow the boot.ini modification instructions.

You will have your XP loader as usual and chain boot into GRUB on (hd0,6). The only thing I see is you need to find a way to store the 512 byte sector under *nix as writing to NTFS is not allowed. May be a floppy.

Another advice, just delete that stage 1.5 thing, I have no idea why they like to use it as it just create more conflicts with other boot loaders.