Is it hard to police? Put stories in to an advertising bidding queue, buy the story you want to advertise in; probably decreasing cost starting at 5x average for the sub (or something like that). Queue items get flushed to the site after a timeout (few minutes). Advertisers police their own ads, they pay up front but can remove ads from a story.

Advertisers is such a system will be rewarded for targeting appropriate subs, they get lower costs, less competing bidders.

Advertisers could even virtue signal on predicted negative stories, "we too think Hitler should have kicked back and smoked a bowl, fellow kids".

Seems like there's a million and one ways to do similar: giving advertisers control (and using timed-bidding to push revenue).

i'm not too familiar with the mechanics of digital ad buying, but i can't imagine any advertiser will be willing to sit there and individually choose which individual stories their ads appear next to. don't ad purchases happen in bulk?

Well, from the limited amount of Google ad buying I've done, and some independent stuff that's pretty much it -- of course most advertiser's would be automating the purchase, and I daresay Reddit could even provide a front-end for that. Social media managers (manipulators) are a pretty common position in many companies -- they're already "optimising" story postings, timings, comments, and such. If you're already managing the story submission it's not much extra to manage the advertising.

Choosing where to advertise by human hand seems like a great way to maximize the effect of a given budget.

I wonder if there is a corollary to rule 34: that someone, somewhere, will find anything objectionable.

Typical HN pessimism, so objectionable. /s