If you *really* don't need anything else, a viable approach would be with an advboot-like tool like what @pazos came up with on
Cervantes.
You'd still have to plug that somewhere in rcS to short-circuit the boot-process, so, again, this falls in the "fragile & intrusive" side of things
.
(Also, you'd first have to adapt the tool to Kobo devices, which shouldn't be too hard, as the hardware is fairly similar
).