If I got you right, you can do smth like this:

First, create tasks as it was mentioned in productFlavors section:

all { flavor ->
    task("doSomething${flavor.name.capitalize()}") {
        doLast {
            println "doSomething${flavor.name.capitalize()}"
        }
    }
}

Then add this code to the end of the build.gradle file:

afterEvaluate {
    android.productFlavors.all { flavor ->
        tasks."assemble${flavor.name.capitalize()}Debug".dependsOn "doSomething${flavor.name.capitalize()}"
    }
}

In this case every assembleFlavorDebug task will depend on the corresponding doSomethingFlavor task.