The first guy I remember seeing an exaggerated open forehand stance from was Kevin Curren. I remember Bud Collins saying it looked like he was firing forehands while astride his trusty steed or some ridiculous Bud-ism like that. While known for the serve, Curren certainly had a heavy forehand and he took some big rips with a very wide stance.
His opponents did everything they absolutely could to get the ball to his backhand which wasn't a bad shot, but his confidence was fragile on that side. Curren is one of those guys I thought had the talent to be great but lacked the confidence to impose his will on guys like Mac and Boris.