Long prior to Last Man Standing this same technique was in use, an X in the suffix.
In the Hardy Boys book "The Shortwave Mystery", published March 1945, Frank Hardy originally had callsign VN16Y and his friend Chet Morton was VY84Y. These callsigns did not match the callsign format of the US at the time and were completely fictional. I might add that at the time the book was written and published there were no legal private Ham transmissions, due to wartime restrictions. Hams were not allowed back on the air for 8 more months after the book was published.
Sometime later the callsigns were changed to match the real format of US ham callsigns, with Frank's changing to K2XEJ and Chet being K2XOB. At that time X, Y, and Z were not allowed as the first letter of the suffix.
In the 1966 version of the book Frank's call changed to WB2XEJ, a callsign that cannot be issued to a ham. However, Chet's call was W2RBR in the same book, as far as I know a 100% legal callsign.
However, I have to wonder. If a writer today uses a legal but not currently assigned callsign, and later that callsign is assigned to a user, how is the writer at fault? If I invent a totally new name, one never used before, and have this person as a villain in a work of fiction, and later someone names their child that name, no case could be made that I was describing that person in my work.
To take it one step further, look at the Hardy example. When the second set of callsigns were used in the book they were NOT legal callsigns, and yet today a 1 x 3 with X starting the suffix would be legal. If you use a currently void callsign, such as a 2 x 3 with an X as the first letter of the suffix, who is to say at some point the FCC won't issue that?
T!