It depends what you mean by sending notifications.
Flarum has notification drivers. By default there's web and email. Additional drivers can be created by extensions using the Flarum PHP extension API.
If you want to broadcast the notifications of an existing driver, you could read the existing notifications and forward them to a different API. This could be achieved via the REST API by reading the GET /api/notifications endpoint but you would need to make one request per user, and poll on a regular interval, which likely isn't an optimal solution.
Using the PHP extension API you could hook into Flarum to react to a new web or email notification being triggered, or register the new notification driver with a completely independent implementation from web and email.
