Loading the manifest file is already done lazily, but when the file changes, not reloaded. The website must now be restarted to pick up the changes.
Adding a ReloadOnChange boolean property to the WebpackConfig class with a default value of false would keep it backwards compatible and an opt-in feature per manifest (and in line with ASP.NET Core conventions for configuration files).
The reloading could be done by using a FileSystemWatcher and subscribing to the Changed event.