There isn’t a command line prompt to lock the database, but I could see it being helpful. I created https://liquibase.jira.com/browse/CORE-1980 so the feature doesn’t get lost.
For now, you could call out to the java APIS: LockServiceFactory.getInstance().getLockService(database).lock() or something similar.
Or, you could create the databasechangeloglock table yourself and set the LOCKED column to 1. That is all liquibase uses to track the lock.