LILO is perhaps the best known Linux boot loader and many people use it to single or multiple boot OSes. What is less well known is that LILO can also boot from disks that do not have any Linux partitions at all. This follows from the design of LILO. The installer stores lists of disk blocks to read in and the boot time portion does not care what kind of partition the boot images are stored on. All that is needed is a way to run the installer on a non-Linux partition. Any filesystem supported by Linux can be handled, provided it can be mounted RW. This includes all the FAT based DOS and Windows filesystems.
What I describe below is not actually Etherboot specific. It will work just as well if you want to boot a Linux kernel from a DOS partition. It just so happens that Etherboot can build images that look like Linux kernels to the LILO installer. Therefore most of the cleverness is in getting a LILO installer to run on a non-Linux partition.