why don't you write one?
He could write one as fanfiction, but he couldn't get it professionally published or legally sell it.
Are we forgetting all those books where that exact thing has happened? Fans who wrote Trek or Wars books and published them, despite being nothing more than fanfiction? Amazon certainly doesn't seem to care whether something is licensed or not, until Paramount/Lucas finds out about each specific incident and complains. Until then ...
Granted, the reviews of each of those have been pretty dreadful.