.gitkeep is intuitive and easy to understand. Unignoring a .gitignore is not intuitive. This falls squarely into "clever optimization tricks that obscure intent and readability". Don't do things like this.
It's not that hard to update a .gitignore file every now and then.
Then put a comment in the .gitignore.
Using the actual tools built in to git directly removes steps in the process, which is always a good thing, it's documented as part of the git documentation, so you don't have to create a wiki page explaining why there is a ".gitkeep" file that git doesn't recognize itself.
Saying "It's not that hard..." is fine for projects with a few contributors but does not scale.