Obviously, one could not be adopted today. It would make the US look too much like a "banana republic." (Remember the ridicule of Nixon's short-lived "Ruritanian" uniforms for the White House guards. http://cdn.billmoyers.com/wp-content/uploads/2013/08/white-house-secret-service-uniforms-nixon.jpg) But is there any time it could have been adopted? Maybe at the very first inauguration (of George Washington)? True, some would see therm as too monarchic or militaristic; but there were some people who wanted more pomp in the presidency those days. (Some wanted a title like "His Excellency." Just calling him the president seemed too common; as John Adams observed, "there were presidents of fire companies and of a cricket club." Of course others, who thought that any title not provided in the Constitution was inappropriate, ridiculed the plump first vice-president as "His Rotundity.")