Looked through https://github.com/WebDevStudios/CMB2-Snippet-Library and didn't see a snippet for this. CMB2 says it supports a object_type => array( 'comment', ) but I don't see the functionality built into the code. Is there a way to do this with filters?

Adding the functionality would not be hard. A dozen lines in CMB2_Hookup.php in function admin_hooks() would do it. So I think its safe to assume this was an intentional exclusion?