Here’s a dumb idea:

Give people the ability to submit a “Show HN” one year in advance. Specifically, the user specifies the title and a short summary, then has to wait at least year until they can write the remaining description and submit the post. The user can wait more than a year or not submit at all; the delay (and specifying the title/summary beforehand) is so that only projects that have been worked on for over a year are submit-able.

Alternatively, this can be a special category of “Show HN” instead of replacing the main thing.