def utilityMethod() {
    doSomethingWith(project) // project is magically defined
}

import org.gradle.api.Plugin
import org.gradle.api.Project

class FooPlugin implements Plugin<Project> {
    void apply(Project target) {
        target.extensions.create("foo", FooExtension)
        target.task('sometask', type: GreetingTask)
    }
}
class FooExtension{
    def sayHello(String text) {
        println "Hello " + text
    }
}

task HelloTask << {
    foo.sayHello("DOM")
}

c:plugintest>gradle -q HelloTask
Hello DOM

target.ext.utilityMethod = SomeClass.&utilityMethod