if you are wanting to boot from the usb device, then the root of the os has to be on the usb device.
the system would need to boot from hda .. but not the gentoo / the gentoo /boot as it's own partition, then install the lfs kernel into there, after building onto the usb device.
when you want to boot into the lfs system the device would have to be plugged in and the lfs kernel selected.
btw, this is a limited usb install this way, it would not boot on any other machine so it wouldn't be usefull as a rescue media.