Currently there is setInitialRoute/pushRoute which has a timing sensitive API that could use improvements.

Simultaneously, there's asks in add-to-app to be able to send generic data initialization while loading up new isolates that's richer than sending a string to define the initial route or choosing a vm entry point. #23786

Simultaneously, in full-app cases, we need to populate the isolate initialization with iOS 13 color data.

Simultaneously, it would be nice to also be able to develop/debug add-to-app cases by just running the wrapper as a full-app using mock initialization data to do lighter development as @blasten suggested.

Perhaps something akin to application:(UIApplication*)application willFinishLaunchingWithOptions:(NSDictionary*)launchOptions's launchOptions NSDictionary.