theoretically simple (never tried it myself yet lol) throw my rootfs onto your memory card as per howto and then copy the kernel zimage to your deivce as normal (store it on a ext filesystem somewhere like in /opt etc) and then i think all you would need to do is :

kexec -l /opt/zImage

you may need to then run

kexec -e

dont have a clue if that will work or not though