I have a property defined like this:
${project.parent.basedir}
I don't think there is a non-programmatical way to do that. So I suggest a groovy one-liner with the Maven GMaven plugin (GMaven is usually the simplest way to embed programmatic code into a pom):
org.codehaus.gmaven gmaven-plugin setproperty validate execute pom.properties['main.basedir']=project.parent.basedir.absolutePath.replace('\\','/');