Do you intend to be doing this with some regularity or want it integrated with Discourse for any other reasons?
If not, the simplest approach might be to just roll an n sided dice and then count down the replies. You probably don’t have different sided die to infinity lying around but you can pretend you do with RANDOM.ORG - Integer Generator
Generate 1 integer, value between 2 and however many posts there are including the first, then get numbers. To the right of the posts you can drag the slider to the result number and it will highlight the post.