Generally speaking in the Navy and Marine Corps we're discouraged from representing or using our rank to influence the public or gain favor. I don't go around calling myself by my rank, I'd expect a former Rear Admiral not to as well.
In the Marine Corps, honorably separated Marines are entitled to use their rank as a courtesy title in civilian life. Wearing jewelry to celebrate and pay homage to one's service is hardly influencing the public.
I've seen pendants with an EGA, spearheads, and whatnot but not insignia. I kind of diluted this thread by talking about rank instead of insignia at first. Very different things.