Overriding module properties from parent POM in Maven

Only way I know to do this for sure is to define the property on the command line, e.g. mvn -DsomeProperty=strongValue <mvn goals/phases here>.

You might experiment with profiles defined in the parent; I suspect that won't help.

No, you can't. The idea is that if it shouldn't be possible to override a value, don't use a property. If you have no other option, you might want to force it with http://maven.apache.org/enforcer/enforcer-rules/requireProperty.html which will break the build if a property has a different value than expected.