I came to this post as I am doing my research right now in crafting a proposal for exactly the solution you are recommending. I have highlighted your point :

artificially promoted content is already mixed into trending, by voting bots.

My intention is to bring a complete solution in a proposal to replace the existing 'promoted' tab as you have the same sentiment, is poorly implemented and inefficient as hell.

What I also what to bring light to, is that all of this rewards being funneled into bots could have been burned to lessen the inflation load on the Steem token.

More so than that, I would like to link my proposal with @jerrybanfield's project proposal whitepaper wherein the funds generated from a system that you have outlined, shall then be directed towards funding projects for the improvement of Steemit itself.

I am currently sourcing all of the important thoughts and points, then moving into the actual mechanics of how it would work.

I am intending to adapt a solution based on Google Adword's auction model mixed in with Facebook Ad's Optimized CPM model, where the higher your Impression to Click-thru ratio is, the lower your cost. As such incentivisizing promoters to focus on garnering impressions by targetting users who will be most likely to be interested in their post.

The promoter should have the ability to have certain function of targetting, choosing specific topics, or even down to users which have engaged with a post of the same topic from another author, or a list of authors even, to see the post, therein increasing the laser targetted focus on who would most likely want to see that.

A system as such would more or less negate a majority of user who utilize the likes of upvote bots as it provides a better platform, all while having the funds they are spending go into projects that may very well aid in the growth of Steemit as a platform as well.

Do you have any thoughts you'd like to add to my rough thought process so far? I think you have outlined essentialy 75% of what I was intending to propose.

God SPeed Brethren.