Not exactly on topic, but we almost did a version of this, at the suggestion of our realtor.
As first time home buyers, we were unsure of what we really wanted. After looking at 50 or so houses, we finally narrowed it down to two specific neighborhoods in my city. Houses in these neighborhoods are built well, offer good bang for the buck, and are in desirable locations, so if they go on the market, they're usually snatched up on the first day.
Since we narrowed it down to these two specific locations (location location location being our top priority), we were about to post flyers on every single door in those two neighborhoods saying "if you're planning on selling, let us know!" Our realtor's thought process was that it could either do nothing, or it could connect us with someone getting ready to sell and save them the trouble of finding good qualified buyers, or listing on the MLS.
A house came on the market the next day in our top neighborhood that we ultimately ended up purchasing, but we were ready to go for the whole "knock on doors/put a letter in the mailbox" thing.