Are there any circumstances in which the terms 'moral' and 'ethical' can be used interchangeably?
No, at least not in good form.
Morals are principles concerning the distinction between right and wrong or good and bad behavior.
Ethics are the set of moral principles that govern a person's behavior or the conducting of an activity.
You can be ethical because you follow your morals, but your morals and mine may differ, so although we appear amoral to each other, we are both still ethical.
Bad form is when people say "that is unethical" when they really mean it's amoral, the only reference where that statement is valid is among colleagues who all share the same ethics.