How can I enforce maintenance_mode at all times in a specific environment? In Drupal 7, we could override the $conf variables in settings.php to force a site (e.g. development) in maintenance mode. In Drupal 8, this has changed to $config, but the maintenance mode variable is not part of the configuration anymore. How can I force system.maintenance_mode in a specific environment's configuration?
I think you might want to take a different approach.

The maintenance mode is now a state in Drupal 8, not a configuration. The difference is a state is a ephemeral state that's specific to the current environment (see the state API docs for more info).

As such, you might want to replace the State service with your own service class that uses the StateInterface interface if you're looking to fundamentally alter the way state behavior (which could lead to unforeseen problems).

Another tact might involve some sort one-time toggling of state (e.g. programmatically as shown above, a drush state-set / drupal state:override) based on some event (e.g. deployment of site).