It's a great implementation, first and foremost.
How I would do it
let people add their posts as promoted, but take all "promo" votes/money and burn it.
As I understand it, paying for promo is a nice boost, but you can use it without paying a cent, and if your content explodes it's no better than hot or trending.
So, make @null part of multi sig, and if the author decides to promote content, burn all of the generated money.
Makes sense?