I don't really understand why this is a problem...
Maybe you could try to use the <skipAssembly> tag in your plugin
configuration at the parent level like this;
<plugin>
<artifactId>maven-assembly-plugin</artifactId>
...
<configuration>
<skipAssembly> ${assembly.skip}</skipAssembly>
....
</configuration>
...
</plugin>
and set a property in your parent POM, for example:
<project>
...
<properties>
<assembly.skip>true</assembly.skip>
</properties>
...
<project>
and set the same property in your children poms with a "false" value.
I have no idea if this is going to work, try it out!!