Вероятно, этого можно добиться, объединив плагин maven-dependency-plugin: copyDependencies с плагином сборки.
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-dependency-plugin</artifactId>
<executions>
<execution>
<id>copy-dependencies</id>
<phase>process-resources</phase>
<goals>
<goal>copy-dependencies</goal>
</goals>
<configuration> <!-- by default all scopes are included -->
<!-- copy all deps to target/lib -->
<outputDirectory>${project.build.directory}/lib</outputDirectory>
</configuration>
</execution>
</executions>
</plugin>
<plugin>
<artifactId>maven-assembly-plugin</artifactId>
...
</plugin>
Ваш дескриптор:
<assembly>
<fileSets>
<fileSet>
<directory>${project.build.directory}/lib</directory>
<outputDirectory>/</outputDirectory>
<includes>
<include>*.*</include>
</includes>
</fileSet>
</fileSets>
</assembly>