Select maintenance subplans via database_id
Maintenance Plans are actually SQL Server Integration Services packages stored in msdb.  

You can see the binary package by looking at msdb.dbo.sysssispackages.  

You cannot easily modify the package using T-SQL, nor can you easily inspect the package contents to determine which databases are being backed up.

Instead of using Maintenance Plans, it is advisable to use SQL Server Agent in combination with some pre-designed set of scripts, such as those provided by Ola Hallengren or MinionWare