quickly browsing over the thead, it appears to me that
you want to boot a kernel which runs from the USB stick
on a system, where the BIOS cannot directly boot from
the USB.
If this is correct, you should find some space on a device,
which is accessible from the BIOS, where you put the kernel
(and maybe an initial ram disk). This maybe the device where
Grub itself resides or anything else. Then you can direct
Grub to load the kernel from the other device and boot it.
This is in some sense similar to saving a master boot
record from a device and telling Grub to chainload it
(a nice way to test new MBRs).