Honestly, dermatology requires a fairly different skill set and vocabulary - most FP grads transitioning to derm would essentially be starting from scratch just like the new residents fresh out of internship.
I've interacted with a couple of DO derm residents, all of whom were previously boarded in FP. The impression I've gotten is that a lot of DO derm residencies are based in community private practices, where the residents are essentially used like PAs/NPs, but at a fraction of the cost of hiring a midlevel. Regular residents have to staff every patient with an attending. However, because these guys are board certified physicians (albeit in a different specialty), they can see patients independently, without an attending present, and bill for it. Some of these positions sound more like a money-making scheme for the PD/self-study program than a residency position.
Again, this is just the impression I've gotten from my admittedly limited exposure to the world of DO dermatology; YMMV.