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 part of multi sig, and if the author decides to promote content, burn all of the generated money.
Makes sense?