Certainly some of the earliest WSOs trained in UK for RSAF Tornados were qualified pilots. I met one lad at Finningley in late 1986 who I had sent first solo nearly two years earlier. The RSAF at the time of signing Al Yamamah 2 had no back seaters at all, at which point of course BAE immediately said 'we will train some for you - give us even more money'.
I believe the initial selection for the back seat had a lot to do with (lack of) family clout and royal influence.