I don't think app submodules are a good idea, they would complicate things a lot without adding much (if any) benefit: Instead of just forking the BangleApps repo, you first need to learn about submodules, then fork/create the app repo. And then fork the BangleApps repo anyway to update the submodule. (Not to mention the complications if an app maintainer seems inactive/deletes their repo, or for global changes which affect multiple apps)
CODEOWNERS looks interesting though, it could be a neat way for people to opt-in to receive notifications about certain apps, and you could ask issue submitters to look at that file for people to @mention. It seems a bit duplicate with the per-app readme/blame, but it also means people who for some reason don't want to be bothered anymore could just be removed from the CODEOWNERS file.