I love the concept and love the fact that the plugin works.
The job configuration input requires a log of information. My preference would be if you had a mechanism on the global configuration to redefine database connections that jenkins jobs would refer to. https://wiki.jenkins-ci.org/display/JENKINS/Publish+Over+SSH+Plugin is an example of a jenkins plugin that has a similar issue. That feature would reduce the job parameters from 9 to 4 and would allow users to configure liquibase conversions without having direct access to the passwords.
I haven't looked at the code but it looks like the plugin is using jenkins' ${PATH} to find the liquibase executables. My preference would be to configure this at the java classpath level. That way liquibase can be maintained independently of the jenkins server and we could run multiple liquibase versions with the same set of database drivers.
Again, this plugin is a great idea and the notion of having easy to read jenkins badges for the conversions is fantastic. Please let the forum know what changes you are considering and people may volunteer to help with them.