Preferential voting hack

I've been vaguely following the US elections, which always leaves me with a sense of incredulity about the two-party system there as enforced by their lack of preferential voting. In many other countries, including Australia, you vote for multiple candidates in order, so you don't have to worry about whether your preferred candidate has the votes to win. If they don't, your votes are automatically redistributed to the next candidate according to your preferences.

Without preferences, you often suffer the spoiler effect, where a third-party candidate not only can't win, but actually hurts their own cause by running because they divert votes from their nearest political allies. Of course, this is a widely known problem covered in much more entertaining detail in CGP Grey's video on preferential voting. Despite this, there's very little motive to change the system because the people in power lose the most by changing it.

However, maybe the will of the incumbent political parties isn't necessary to change the system. Much like in my Copyright 2.0 idea, you can sometimes implement a new and better system on top of the flawed old one. Runoff voting works by repeatedly eliminating candidates, but there's no necessity that all the eliminations happen at the same time. In fact, the US presidential election could be thought of as the last step in a runoff voting system where all the other candidates have already been eliminated.

So what if all the independent parties got together and held one big preferential pre-election primary? Any registered voter could cast a vote for any party, just like in the real election. You'd do a proper preferential runoff calculation, and parties that were eliminated in this primary would drop out from the actual race. Once you're down to the final two parties, they contine on to the election. Smaller parties would want to be involved because it gives them a feasible shot at the election. Once established, the bigger parties would want to be involved because it's in their interest to win votes and eliminate the smaller parties.

It would be a pretty substantial undertaking logistically, but I can't see any theoretical reason why it wouldn't work. I think there are a lot of people and groups who want to see the US transition to preferential voting, including those with the resources to make something like this happen. Up until now the main dialogue has been about how to push that change through the existing political process, but maybe the better path is to go around it.