Yes, people put up a post asking for feedback or announcing they've launched. Usually its takes the form of "Ask YC: What do you think of my start-up idea?" or something along those lines. It helps not to be spammy about it, and sticking around to answer questions is always a good idea. So is having a tough skin, because people will question your every move.

Some recent examples: http://news.ycombinator.com/item?id=267640

http://news.ycombinator.com/item?id=267049