Mason would not be able to compete with the work rate of Haye for 12 rounds. And although Haye might be skittish for six rounds with Bruno, he is perfectly capable of coming from four/five rounds down early to either get to Bruno late, or win a close decision.
Mason as a better work rate than Haye, Haye doesn't have a work rate at heavy, infact he wasn't exactly a workhorse at cruiser. I'll meet you half way with the Bruno thing in that Haye could do something with Frank after 8 rounds but Frank as better boxing fundamentals as oppose to Haye's flashiness, Bruno's jab is no joke and Haye's low punch output wouldn't be too taxing on Bruno. I watched Bruno/Witherspoon again recently and was surprised by Frank's quickness and punch variety, something which i didn't remember and is easy to forget after the robot jibes aimed at him. Bruno and Mason are strong HW's, Haye isn't. He's a flashy pot shotter who's KO power at Heavy i think is still up for debate. He ain't dropping Mason and if he reached 8 rounds with Bruno i reckon he'd be blowing out of his arse just as much as Frank. Haye as decent head movement can look dynamic and closes the range quickly, if somewhat amateurishly, and he smothers his own work. But really he's a glorified slugger who doesn't use proper boxing fundamentals and has poor stamina, hence his concsious decision to keep a low output. Bruno's stamina issues come from his muscles and not knowing when to ease off. I see both stopping Haye, if not Haye would be hard pushed to get a decision.
Sure Haye could and was lazy sometimes, but look at Mason. He looked like he might blow the Biggs fight he was that slack, and the Martin and Wills fights were hardly evidence of Zack Padilla like work rate. As long as Haye respected Mason's power, I suspect he would of outpointed him.