In fact this property is only used to generate the xml report but not to specify the memory to be used by the forked Surfire process. As we have never encountered such memory limitation, I'm closing this issue with resolution "won't fix".
ReportTask task = new ReportTask();
task.setMaxmem( maxmem );
task.setOutputDirectory( outputDirectory );
task.setCompileSourceRoots( getCompileSourceRoots() );