...No, I don't think so.
I think just stabbing my son (probably multiple times) to get a meeting with me, would probably put me off ever working with Rocks.
Rocks might end up being completely sincere, but he does kind of come across as a con-artist of a sort. There's just a lot of red flags surrounding Rocks that would put off a King from working with him directly. His "first mate" Whitebeard never looks happy to be around him. He's a pirate with a crew made up of gangsters, and cult leaders, and thieves. He's always going on about this "big score" he's planning.
The World Government isn't perfect. And changes need to be made. But, putting Rocks on the throne, instead? That'd sound like a recipe for disaster.
I do wonder if Rocks ever told Harald about Imu, though. Or even just his crew. Did Rocks keep that little nugget of information as a ace up his sleeve for later, or did he tell people about that?
Plus, Harald's pretty busy. Arguably, too busy to go off on a mad "get rich quick" scheme. He's trying to get his own house in order. Trying to change Elbaph's culture and negotiating peace treaties with other countries. Harald's time is better spent building his own country up, rather than trying to claim the World.
Plus...Harald's a little bit of a chicken, in some ways. I don't think he'd have the guts to actually rebel against the World Government in public. Harald let his people pressure him into marrying a woman he didn't love. He let them pressure him into spending all his time trying to build up the country so much that he kind of neglected both of his sons. Harald seems VERY susceptible to peer pressure, despite his rebellious days as a youngster. I don't think he'd be willing to rebel against the World Government. Especially since he seemed to have been working really hard to try to JOIN it, in order to get into the Reverie with the other kingdoms, in order to support trade relations.
So, no. I probably wouldn't have joined Rocks, even in Harald's shoes. Too much risk. Harald probably knew that if the King of Elbaph joined with a pirate, that would make his entire kingdom a target of the World Government. And that's just not worth it. Maybe that was a mistake. Maybe Rocks being the new King of the World would have been a improvement. I tend to doubt it. But, Rocks does seem more reasonable than I assumed he would be. But, given the information he had, I think choosing not to join Rocks was probably the right call.