I noticed that Ron Glass (and respectfully RIP Mr Glass) from Firefly (playing Shepherd Book) also played the same type of character (Loken) on Star Trek Voyager Season 7, 2 years earlier in the episode "Nightingale".
Did he play it so well that Joss Whedon cast him in Firefly, or was that role the inspiration for Shepherd Book?
I've scoured the internet for references to Joss Whedon's casting decisions, and the closest I've come is this Trek Today article, which references an unavailable Hollywood Reporter article.
Ron Glass, who appeared in Star Trek: Voyager's 'Nightingale,' is best known for his role as Detective Ron Harris in the 70s comedy series Barney Miller. The veteran actor has featured on series such as The Twilight Zone, The Practice and The Education of Max Bickford. It is likely Glass will portray Book, an older, religious man, out to see the universe and spread the word of God. Book will reportedly appear in only ten out of the first thirteen episodes.
I believe they only mention Star Trek due to that being the sites focus.
Further, in this memorial article, Whedon doesn't reference any of Glass' previous roles.
It looks like, unless anyone can access the Archived Hollywood Report article of the firefly casting, or contact Glass' former agent, it's unlikely we'll have a definitive answer, but it seems like 'No'.
