SCOM - Enable "Database Backup Status" monitor for select databases

The process is as follows (assuming you are using the Console to create the group):
Create a custom group; give it an appropriate name that ends with "Group".
Example: "Spinco Production Databases Group".
Define the custom group to dynamically include either the workflow target type: MSSQL on Windows: Database (Microsoft.SQLServer.Windows.Database) or a child class of the target type or the class type that hosts the target type; in this case the hosting class is: MSSQL on Windows: DB Engine (Microsoft.SQLServer.Windows.DBEngine). You can use any properties of the class types mentioned above to define your dynamic membership rule.
Class graphs available here.
Override the monitor for a group; specify your new group as the target.
In which unsealed MP this group definition should be stored is a topic all on its own and is beyond the scope of this response but just keep in mind that the override must be stored in the same unsealed MP as the group.

The best approach for overrides in general (according to this guy) can be found here: https://monitoringguys.com/2016/05/24/how-to-correctly-create-an-override-for-a-scom-workflow/. But unfortunately the guidance at the link doesn't go into detail about groups because it's a bit more complicated to explain.