although in practice I'm a bit skeptical there are enough humans viewing the content right now to honestly justify it

Maybe, if you just consider the possibility for people to read the particular promoted post. If it can also be used as a way to put yourself in front of people who might become followers, that changes the calculation, though. With that in mind, I do think we have enough people here to support it. Also, as I'm exploring with Popular STEM another use case might be for promotion campaigns by communities, instead of just individuals.

I think it would be a problem if people started to think of post promotion as a lottery ticket for getting a big upvote.

I agree with this, but I don't think it'll work out that way. Basically, post promotion means that the author has "skin in the game". Maybe it's a way to improve the odds that a curator will look at your post, but you still have to create something that they'll be willing to vote for. I would think it would create an extra incentive to produce content that is worth voting for.