XOSL is also not tied to any operating system, and is fully manipulable from the graphical interface. In fact, that is the only way to set it up. The "footprint" thing is a non issue as this bootloader as well as others drops out of memory upon loading an OS. It's only problem is that it hasn't been maintained in the past 6 years, and that it requires an old DOS FAT16 partition to be installed onto. I am trying to get it to install onto a USB drive so I can do with it what you have done with grub, which is boot anything, and have anything on your system be available to boot, but have so far been unsuccessful at it. I even tried DOSBox to see if I could fool it into thinking the USB drive was a hard drive. What surprised me the most about XOSL is that it is a graphical interface, but the thing takes little memory space, and disappears after the OS selection is made. Grub 2 page mentioned that it would have graphical interface capacity.
I noticed that you did not have BeOS, OS/2, or DesqViewX in your list. :-]
DesqViewX ran under a QEMM modified DOS, but it was a really nice, early X-server, and would even allow a distributed process to be run on a remote machine. It would be cool to have a modern version of that floating around!

ASCII ya later...