The underlying Liquibase objects are probably the best way to work with objects rather than starting with the XML. I tend to see the XML as a possible serialization format, but not the root of how you should be working with Liquibase programatically.
Liquiface (http://www.liquiface.org/) is an existing open source 3rd party project to make a nicer interface to Liquibase. Datical (http://datical.com) is company building a more enterprise-level application on top of Liquibase which includes a nicer interface for creating and managing changelog as well as features like forecasting changes.
Nathan