You do realize that FJ was asked by Paramount to serve as a consultant of some sort when TMP was being made, because of how successful the TM and associated works had been, and that he declined that offer? On the grounds that he didn't want Roddenberry to feel he was dictating how technology should exist in the Star Trek universe?
According to his daughter Karen, in this interview
of 1999, it was her understanding that the little snippets of FJ's work which were included in the first three films were done by production staff who happened to enjoy his contributions and technical work. He was never given any formal credit for those elements, and his decision to not accept the opportunities as a creative consultant could have arguably hurt him since it allowed Gene and Paramount to dismiss his work later as being "non-canon" stuff.