You can--I remember Bob and I having a discussion about it, wasn't working for me, worked for him.
What I've found is that Ubuntu puts a symlink to their vmlinuz and init files in /. So you can bypass its grub by doing, after specifying the partition
WIthout pointing to its grub. (I might be wrong on the path, the symlinks might be /boot/vmlinuz and /boot/initrd.img.
On the other hand, when Bob and I had this discussion, some time ago, for him, the usual chainloader +1 worked perfectly--might have been a typo on my part, it was awhile ago.