Would it be possible to send a message that would display an in-app in the user mobile?
TL;DR
Developers can utilize in-app messaging through the database to update user preferences and control when alerts are shown. One potential method is to incorporate push notifications using Appwrite. Here's the documentation on sending push notifications: https://appwrite.io/docs/products/messaging/send-push-notifications
it should be fairly doable if you mean by In-App Messaging. its not supported out of the box but you can use the database and update the user's prefs if they have seen/interacted/dismissed the alert to not show it again later.