Hey you've done a great work!
I've assembled a diskless desktop zero noise no fans and would like to boot debian already installed in a usb flashkey all ready to boot, with kernel and grub in his mbr.
The target system doesn't obviously support usb boot, but i've tested that throught plop manager on a floppy, it successfully boots the basic grub shell off a usb pen.
The trouble is that once in the shell, I can't look at the usb disk at all, like it were lunched directly without the plop support (eg. by a grub floppy).
If in the pendrive mbr instead of the shell I put the preconfigured boot loader, a get an "error 21" that usefully stands for "unknow error occured".
So have i missed something or is plop manager that can't chainload another boot loader??