I see that appserver folder resides in the app's home dir. How does it manage the local changes, for example, a .js file? Aren't there default and local versions of it? If I create an appserver directory under [app]/local/appserver, will Splunk understand and give it preference over [app]/appserver?
The following documentation speaks about some of it - About files and directories for apps and add-ons
It speaks about local and shows
Meaning, $APP_HOME/local/
is reserved for configuration files.
A good documentation about the Anatomy of an app
However, it doesn't offer a solution for you...