After doing a bit of research, I think I may have found the answer. For those who don't want to delve, the gist is that EM Rauch is a novelist who started writing screenplays. WD Richter started to mentor him and was intrigued by the BB concept. It seems to me, reading between the lines a bit, that Richter channeled and directed Rauch and that the novel and movie are so entwined as to be inseparable. I think they must be considered two sides of one coin. And they both came out in the same year. What I find most interesting is the differences between the two. A strict novelization would have followed the story slavishly, possibly adding, but not changing. There is a lot in the book that is not in the movie and vice versa. Interesting.
http://www.worldwatchonline.com/earlmac.htm(from Marvel Super Special Vol. 1, No. 33; interview of W.D. Richter by James Burns)
Richter tells how he got involved with Earl Mac Rauch and Buckaroo:
In the early seventies...
...Richter and his wife read a review of Dirty Pictures From the Prom, a debut novel from another Dartmouth grad, in their alumni association's newsletter. Richter bought the book, liked it, and decided to send its creator a fan letter.
Dirty Pictures From the Prom's author was Earl Mac Rauch.
Richter recalls, "I wrote, 'I think you're a terrific writer. If you're not happy doing what you're doing, come to Hollywood and try movies, because there really aren't that many good writers out here.' It turned out that, after Dartmouth, Mac had dropped out of law school, and was selling finance contracts for mobile homes in Texas. He took me up on my offer."
At one of Rauch's first dinners with the Richters, "He told us about Buckaroo Banzai (at the time, called Find the Jetcar, Said The President - A Buckaroo Banzai Thriller - ArcLight). Even though Mac's plot and characters were just in a beginning, sketchy phase, Susan and I were immediately attracted to Buckaroo's irreverence, and the concept of a multitalented hero who's always off on an unlikely adventure. We decided to subsidize a Buckaroo Banzai screenplay. I didn't know if I would direct it back then, though. I wasn't sure of what the project's future would be anyway. As strange as Buckaroo Banzai may seem today, ten years ago, it would have been impossible to explain to a studio how it could be successfully shot and marketed. I realized that funding Mac might just be an exercise in buying a very expensive story that I wanted to read, but my other, perhaps even more important impetus, was that I wanted to help him get started in the movie business."
Consequently, with the Richters' input, Rauch wrote several Buckaroo Banzai adventures. (Including The Strange Case of Mr. Cigars, which, according to Mac Rauch "was about a big, huge, King Kong-size robot, some big secrets, some exotic locales and Hitler's cigars. It was crazy." - ArcLight)
"Mac would get thirty or forty pages into a script, abandon its storyline, and start a new one," Richter says. "The plots involved several different nemeses from our present film, including the infamous Hanoi Xan, Boss of the World Crime League... Many of the characters, like Perfect Tommy and Rawhide, weren't created until later drafts. Members of the Hong Kong Cavaliers that Mac did include, as well as other roles, were eliminated or developed differently. Buckaroo, for instance, wasn't always half-Japanese.
"Mac just thought that the film was too outrageous to ever be sold," Richter explains. "He never showed those partial scripts to a studio, and let his inspirations carry him wherever they led. And, thought Mac is a brilliant writer, he's also a bit unorganized. If I hadn't kept copies of his various Buckaroo storylines, they'd have been lost."