What's a precedent for an online market for downloadable apps, where that market is the primary venue for the platform, where users have enough control over the listings to prevent them from getting swamped with crap (such as individually packaged apps for each Project Gutenberg text), but not enough control to game the system?

There are certainly UI issues. However, I do believe there are precedents for open marketplaces that are not overrun by crap. The digital music businessed of iTunes and Amazon do a good job of being open to any vendor while giving the most popular artists the most exposure to casual browsers.