So, the startup pays some amount, lets say, $500 for the feedback service. HN guarantees that they will receive 100 pieces of feedback using this tool (which of course would be using node.js, closure, haskel, [insert any trending technology here]). Once the 100 pieces of feedback are received, the 'sponsored link' gets removed and another goes in its place. The startup could also request feedback only from users with certain karma levels (which would cost more money).
HN shows a maximum of 1 sponsored review link at the top of the site, or mixed in somewhere, at a time. I dont think it would be too tough to get 100 feedback notes in a single day (with 60k users that love checking out new startups).
Let's say only one is shown each day, thats $15,000 per month in revenue for that service. That'll buy a lot of ramen.