Can I setup a backup migrate schedule from settings.php
According to this thread (the first part is about the Environment Indicator module but the last part is about Backup and Migrate), Pantheon forbids the use of the Backup and Migrate module on their platform.  While it is possible to make Backup and Migrate settings exportable, including for use in settings.php, it's probably best not to violate your web host's policies.

That said, Pantheon should really start writing their rules down clearly somewhere.