Presently there's no way to access the suspension reason within the suspendService() method in modules. Looking at the source in app/models/services.php this is because the updated suspension_reason isn't set until after suspendService() runs, thus the suspension_reason field in the $service object is incorrect (ie it's the old/previous reason, if any).
It would be great to be able to pass this value along to modules which may have their own methods of notifying customers, taking actions, etc.