What, if any, is the distinction between "formal ethics" and "deontic logic"?
Formal ethics comprises more than just deontic logic. Deontic logic is primarily concerned with a logical analysis of notions such as 'ought', 'must', 'may', or 'being permitted', whereas formal ethics comprises all ethical and moral reasoning that makes use of formal methods.
Apart from deontic logic, formal ethics is also concerned with the modelling of formal systems of norms and rules (which are usually conflicting with each other, sometimes also defeasible and context-dependent), the formal modelling of the permissiveness of moral rules as 'soft constraints', dealing with vagueness of norms, laws, and moral rules, game-theoretical explanations of moral norms, formal models of distributional justice, the formal modelling of values including value incommensurability, moral issues of decision making under uncertainty (e.g. the Precautionary Principle), problems of value and preference aggregation, and utilitarian ethics in general.
Some of these topics are also addressed in AI research, formal epistemology, game theory, and economics. Research in this area is fairly interdisciplinary.