I've followed a simpler approach: evaluateBeanshell rule. Thanks to the Stephen Connolly answer, I've managed to find out where the "recursive" boolean was.
<plugin>
<artifactId>maven-enforcer-plugin</artifactId>
<executions>
<execution>
<id>require non recursive flag</id>
<phase>validate</phase>
<goals>
<goal>enforce</goal>
</goals>
<configuration>
<rules>
<evaluateBeanshell>
<condition>false == ${session.request.recursive}</condition>
<message>Non-recursive flag is missing</message>
</evaluateBeanshell>
</rules>
</configuration>
</execution>
</executions>
</plugin>
I think it works in maven 3 only.