Using the suggested Maven properties plugin I was able to read in a buildNumber.properties file that I use to version my builds.
<build>
<plugins>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>properties-maven-plugin</artifactId>
<version>1.0-alpha-1</version>
<executions>
<execution>
<phase>initialize</phase>
<goals>
<goal>read-project-properties</goal>
</goals>
<configuration>
<files>
<file>${basedir}/../project-parent/buildNumber.properties</file>
</files>
</configuration>
</execution>
</executions>
</plugin>
</plugins>