I love git submodules. Also, I hate git submodules. What I love about them is how it enables to you to cleanly compartmentalize dependencies etc. I get the point of havin
Why don't you make changes inside the submodule directory, which itself is a git repo? This way, your app will always have updated library.
Caveats:
You still need to commit the submodule change inside your app repo to put the change in version control (for the app).
If there are more apps than one that are using this library, than this is not going to work, since only one app will be up-to-date at any given time.