I have a property defined like this:
${project.parent.basedir}
Just an update to Sean's answer, I have had to make some minor adjustments in order to adapt it to the latest groovy maven plugin version:
org.codehaus.gmaven
groovy-maven-plugin
org.codehaus.groovy
groovy-all
2.0.1
setproperty
validate
execute
project.properties['basedir']=project.parent.basedir.absolutePath.replace('\\','/');