Best way to configure a allprojects Gradle task from the project it run on?

Question

In the root build.gradle we are creating a task under allprojects that should run before the build of the project that is running the task.

// root build
apply plugin: 'cpp'
all projects {
  task.whenTaskAdded {theTask ->
    if (theTask.name == "build") {
      if (theTask.project.<runPreBuildTask?> == true) {
        tasks.create(name: 'preBuildTask' + theTask.name) {
          // do some stuff
        }
        theTask.dependsOn preBuildTask 
      }
    }
  }
  // other stuff model, toolchains, repositories
}

// some sub project build
// :subProject:subSubProject
// no luck using ext properties
ext.set('subSubProjectDoPreBuild', 'true')
model{
  components {
    ...
  }
 }

For some reference we have nearly 800 sub projects and their build.gradle is being auto generate by a script that converts the existing make file. I realize that we will have to hand edit many of them but for this task we would like to have the root build inject the task into all sub project but only if they some flag or something set. This will give us the flexibility to change the task in the future with out having to edit 100's of sub builds. Please let me know if I am approach this task the wrong way.