No. He couldn't have said no. You can't accurately compare Mick Foley of 1996 to Mark Magnus of 2004. When Foley came into the WWF, he was a somewhat established star, sought out for notable work in WCW, ECW, and Japan. He couldn't veto everything McMahon said, but he could fine-tune things a bit, because he did have enough capital built up by then. Magnus just came up through OVW. Who is he to say "I won't work this gimmick"?
Now I'm not of the whole Bob Holly school of "YOU HAVE 2 PAY YOUR DOOOOOZ!!!" but I think you do need to build up some capital before you start refusing assignments. It'd be like a call-up from AAA telling the manager he's not gonna bunt. Yeah, maybe he shouldn't, because bunting is dumb, but really, shut your freakin' mouth.