One point-Nixon was still alive when Alan Moore wrote Watchmen. Also, it may depend on the celebrity. There are a lot of stories with Elvis as a character, in some form or other. Some that outright say who he is, others that only allude to it. I also want to say I seem to recall reading a collection of shorts where they were all supernatural stories featuring Elvis. Or it could've just been a coincidence that there were at least 2 shorts in a collection. One thing to try is to look up current authors who do this kinda thing, Charlaine Harris comes to mind, or maybe try asking an editor or agent on Twitter, or other social media. Chances are, though, that even they may not know.