Of course he COULD have. He could knock out just about anyone if he landed enough right hands upstairs and left hooks to the body. The question is, if he lands enough perfectly timed and perfectly placed shots to stop Margarito.
I think it's possible Hearns stops him in the first few rounds, considering Margarito's a slow starter and Tommy was at his most dangerous early in the fight. I'd also take Tito to stop Margarito in the late rounds.
I know it is already 110 to 21 in favor of Hearns, but I wanna bump this to get more votes and see if there is any new opinions.