I'll go ahead and do a pull request. Would you prefer the change in 3.1.x or master? 3.1.x will make it available sooner, but it could lead to merge conflicts when you try to merge into master on the next release.
As for the API vs a main (or some other single) method makes more sense for my use case. The API makes for simpler, richer integration for applications that want to use Liquibase programatically, but for a build tool plugin, I like the idea of a single method call because it would be more decoupled.
My goal is for plugin users to have the ability to configure the plugin to use newer versions of Liquibase to take advantage of bug fixes without having to build a new version of the plugin itself. If I'm just calling a single method and using reflection to find it, the plugin would simply work no matter what version of Liquibase was requested.
Steve