Think not planning for that possibility would be reckless. I'd say they both look second choice for OC/FB respectively at worst - and that's an ongoing argument as things stand, who knows how things will play out with fitness and form.
Of course, having a second top class full-back on the books makes Payne to 13 more of a possibility too.
Overall, this makes a lot of sense to me. We are short of full-back cover and short of outside-centre cover if you need Payne to be there. More importantly, we need more experienced quality in our backline when international duty swings round. A worst-case scenario sees 8 backs with Ireland or out injured - Payne, Bowe, Trimble, Gilroy, Cave, Marshall, Olding, Jackson - that would leave us with a ghost of a unit, even if we assume no more injuries. None of those players is a long term shot, maybe Gilroy if he doesn't recover form (but if he doesn't recover form, he's not too much use to us and I think he's doing it). Ludik can be our Nacewa, a versatile high-quality guy who can really shine in that situation and is just a big boost in general.