You would need to run Liquibase programatically (i.e. from your own Java code) and write a class that implements the ChangeExecListener interface. The class implementing the interface would get notified of all changes and would be able to examine the changeset and print out the comment if there was one.
There is currently no way to specify a ChangeExecListener from the command line, maven, ant, etc.
Principal Software Engineer
Datical, Inc. http://www.datical.com/