No, this isn't currently possible. One thing you might want to look into would be to write a script that downloads the latest revision and pushes the code via appcfg.sh. When you do an upload, the script looks for files that have changed (via hashing contents) and only uploads those files.
There's a more complicated scheme where you might be able to host files on Google Storage and have the developer change those files, but appcfg.sh won't push these for you.
--
Ikai Lan
Developer Programs Engineer, Google App Engine
We have a Java app (GWT/GAE) with static content. Usually one developer builds and uploads the app along with the content. But is it possible for a different developer to update the content without having to access the entire war/ directory? Every update to the content is now requiring a new upload from the builder, and it'll be great if we can provide the second developer with a script to upload just the content.