Why not do what is done in the UK and put spending limits in place?
The reason Citizens United was decided the way it was is because it costs money to make movies, and even if you somehow allocate public money towards the distribution of the film on PBS or whatever, you're still insisting that the movie makers aren't allowed to spend more of their money to make the movie to their own standards.
It drives me crazy that people don't understand Citizens United was about free speech. And most people who say they don't like the ruling reverse positions once I explain exactly what it was about. Stopping a film maker from making or airing a political film seems horrible to me.