"Pretty sure this easter egg has remained hidden for almost the past 25 years now! After loading in Map 02 (Plant) and entering in "jb4jh*****" on the password screen and pressing start shows a hidden "Rage Special Credits screen".
Entering "lyjvm*****" and pressing start shows a hidden "Sega Special Credits screen"! (The asterisks can be any value, just as long as the password starts with those characters) That's programmer Jim Bagley on the Rage Credits screen, and Sega Europe employee Ed Hollingshead for the Sega Credits Screen.
More info on Saturn Doom (including that information) can be found at Doomworld Forums."