If I create a text parameter "test" in a Job and configure the Groovy Plugin to call a script with this parameter, it is not expanded:

Groovy Script file: test.groovy
Script parameters: ${test}

Will result in:

<pathToGroovy>/bin/groovy.bat <Workspace>/test.groovy ${test}

System properties like MAVEN_HOME are expanded as expected when passed as ${MAVEN_HOME}.

The same works great with the maven build.