In general I agree with you, as I will get into, but there is one stipulation match that Randy Savage did win, and that was at Wrestlemania X against Crush in one of WWE's first Fall Count Anywhere Matches. That was a pretty unique match at that time, and he won clean. He also defeated Ted Dibiase in some cage matches while champion in 1988. And he did win a King of the Ring Tournament and of course the Championship tournament at Wrestlemania IV So in reality he was probably about 50/50 in stipulations matches during his time in WWE. But in addition to the Retirement Match loss to the Ultimate Warrior, he wasn't ever able to win the Royal Rumble, something Brock did during his first run.
With that being said, I don't like the chances of Randy Savage in this type of match against Brock Lesnar. The way to win a Casket Match is to put your opponent into an unconscious state long enough to put him in a casket and shut the door. I just can't imagine or see Brock Lesnar being in that kind of state to get put in a casket. In fact, I don't think I've ever seen him down long enough for that to happen to him. The one exception might've been his botched shooting star press at WM 19, and even then he got back up and still beat Kurt Angle. But even from chair-shots, Hell in a Cell Matches, Falls Count Anywhere matches, he's went down long enough for a 3 second pinfall, but it takes far more than 3 seconds to lock someone in a casket.
Randy Savage on the other has been dominated enough in matches to where he's been down that long to be put in a casket. The Retirement Match at WM 7, he was out for about 3-4 minutes before finally getting up. If Lesnar can put Savage in that kind of condition like Warrior did, he wins, simple as that. And Lesnar could easily do that.
Taking kayfabe out of the situation and talking about who's the greater all-around pro wrestler, it's Savage easily. As great as Lesnar is, he does not compare to Savage all around. Lesnar is a great worker, Savage is one of the best ever. Lesnar is not so good on the mic. Savage is one of the best ever. Both were big, huge draws. Kind of equal there. But Savage was far more consistent and had a longer career. Much longer. Accomplishments are comparable. As Lesnar was around in an era where the title switched hands far more, and yet he doesn't have that many more championship reigns over Savage. But he was able to win the Royal Rumble, something Savage wasn't ever able to do, and he ended the Streak, though Savage never got a match with Taker at WM. But on accomplishments, they're pretty close. But still, Savage is the overall greater wrestler, and his legacy at this point is stronger I feel.
But with the gimmick match portions of the tournament, overall greatness isn't the way to go really, it's more about kayfabe and who would win. From that perspective, I can definitely see Savage getting Lesnar down for 3 seconds to win a match by pinfall. Plenty of people have done that from Triple H, to John Cena, to Undertaker, to Goldberg, to Kurt Angle, to even Eddie Guerrero. Savage could drop the flying elbow and put Lesnar down for the 1-2-3. But this isn't a regular match. This is a casket match, I just can't see Savage getting Lesnar in such bad shape that he's knocked out long enough to be put in a casket and have the door shut. Lesnar, on the other hand, I have no doubt could.
I really like Brock Lesnar right now, I think he's great for pro wrestling. But Savage is one of my all-time favorites. But to be completely honest, I just can't see Savage winning this match. In the end, I have to vote Brock Lesnar. Though I'm not gonna try to argue that everyone else should. I'm fine with either result. But I have to vote Lesnar here.