How about the Hohenzollern-Sigmaringens somehow get to become the Kings of Prussia? Dunno how, or why, but then you at least have the Catholic Kings of Prussia in place for an election...
Then you'd have the problem of the fact that Brandenburg-Prussia is Lutheran.
I could see a Hohenzollern being put in charge of someone's puppet Poland-Lithuania; possibly a more successful Swedish Deluge leads to some ATL younger brother of Frederick William being placed on the Polish-Lithuanian throne. This could also help greatly in pushing through reforms to make the crown hereditary, centralize authority, etc.; no one's going to vote no if there's a Swedish army sitting outside ready to march in. The Swedish motive would be making Poland a strong allied state against Russia.
Alternatively, you could have the same scenario as above, but with a Vasa instead of a Hohenzollern being placed on the throne initially, but with the Vasas and Hohenzollerns merging. That has the advantage of merging the two countries together when the houses merge. There would certainly be a War of Succession, but beside that, it's workable. You could certainly have a Hohenzollern on the throne of Poland by 1700 or so, assuming a POD in the Deluge.
That does butterfly the Hohenzollerns from taking the title of King in Prussia, but they won't need it if they have Poland-Lithuania.