how to generate a maven property and use it later, in the maven-resources-plugin?

Question

In my project I want to generate a property (essentially a username:password string, base64-encoded) and then use it when the maven-resources-plugin is executed.

The property is correctly generated and visible outside the plugin (I've already tested that):

<plugin>
  <artifactId>maven-antrun-plugin</artifactId>
  <executions>
    <execution>
      <id>encode-user-pass-base64</id>
      <phase>initialize</phase>
      <goals>
        <goal>run</goal>
      </goals>
      <configuration>
        <exportAntProperties>true</exportAntProperties>
        <target>
          <script language="javascript">
            <![CDATA[
              ...<code here to generate base64data>...
              project.setProperty("MAVEN_NPM_AUTH", base64data);
            ]]>
          </script>
        </target>
      </configuration>
    </execution>
  </executions>
</plugin>

Now, the problem comes when maven-resources-plugin is executed: all the properties in my resource file are replaced correctly except MAVEN_NPM_AUTH.

Any idea why exactly this property is not replaced ?