Does $E=m{c}^2$ Require Relativity?

It is universally believed that with his 1905 paper “Does the inertia of a body depend on its energy content?” Einstein first demonstrated the equivalence of mass and energy by making use of his special theory of relativity. In the final step of that paper, however, Einstein equates the kinetic energy of a body to its Newtonian value, indicating that his result is at best a low-velocity approximation. Today, several characters debate whether a mid-nineteenth century physicist, employing only physics available at the time, could plausibly arrive at the celebrated result. In other words, is Einsteinian relativity necessary to derive $\text{mc}E=m{c}^2$?