Custom Notifications in AWS User Notifications

Yes, you can set up AWS User Notifications for custom sending, though there are some specific considerations for your cross-account scenario.

AWS User Notifications allows you to create notification configurations with advanced filtering options. While the service does add service and event type to event rules by default, you can create more sophisticated filters using customized JSON event patterns.

For your specific use case involving AWS Backup failures across accounts, you'll need to implement a cross-account event routing solution:

1. In your backup accounts, set up CloudTrail to log AWS Backup activity
2. Create an EventBridge rule in the backup accounts that captures the backup failure events
3. Configure this rule to send events to a custom event bus in your User Notifications account
4. In your User Notifications account, create an EventBridge rule that captures events from this custom bus
5. Set up a notification configuration in AWS User Notifications that uses an advanced filter to match these events

For creating the advanced filters, you can use the EventBridge console's rule builder to help construct and test your JSON patterns before implementing them in User Notifications.

When setting up your notification configuration, you'll need to:

1. Navigate to the notification configurations page
2. Choose "Create configuration"
3. Select the services you want notifications for
4. Use advanced filtering to specify the exact event patterns
5. Configure your preferred notification channels (email, Slack, Microsoft Teams, etc.)

This "daisy-chain" approach you described (backup account → custom event bus → default event bus → User Notifications) is a valid architecture for cross-account event routing, though it does add complexity to testing and troubleshooting.
Sources
Using AWS User Notifications with AWS Control Tower - AWS Control Tower
Manage and view your AWS Health notifications in AWS User Notifications service | AWS Cloud Operations Blog
Filtering event rules using customized JSON event patterns in AWS User Notifications - AWS User Notifications

There isn't an option to setup notifications in User Notifications for custom events, only AWS events. However, AWS Backup service events are available for setup through the standard AWS event bus.

You can route events from account A to account B's EventBridge event bus and than setup your notifications in account B.

Resources:

• Routing EventBridge events across accounts
• EventBridge backup events

I don't completely follow what you are asking. It is possible to get notifications for backup events using AWS User Notifications. It is now theoretically possible to do this across an organisation, but User Notifications have been broken for me since this update. It may work for you.