I'm not sure I understood everything (from where does this lib directory come from, when do you want the copy to happen exactly?) but the Maven Dependency Plugin might help, its dependency:copy goal takes an outputDirectory per dependency. Below the sample taken from the Usage page:
<project>
[...]
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-dependency-plugin</artifactId>
<executions>
<execution>
<id>copy</id>
<phase>package</phase>
<goals>
<goal>copy</goal>
</goals>
<configuration>
<artifactItems>
<artifactItem>
<groupId>[ groupId ]</groupId>
<artifactId>[ artifactId ]</artifactId>
<version>[ version ]</version>
<type>[ packaging ]</type>
<classifier> [classifier - optional] </classifier>
<overWrite>[ true or false ]</overWrite>
<outputDirectory>[ output directory ]</outputDirectory>
<destFileName>[ filename ]</destFileName>
</artifactItem>
</artifactItems>
<!-- other configurations here -->
</configuration>
</execution>
</executions>
</plugin>
</plugins>
</build>
[...]
</project>
Change the lifecycle phase binding according to your needs.
But as I said, this answer is more an hint, maybe using a custom assembly would be more appropriate. If it doesn't help, please provide more details.