Temporarily change an sbt setting and execute task

build.sbt

val greet = taskKey[Unit]("Says hello to you")

// Reverses the value of `name` and calls `greet`
val greetReverse = Command.command("greetReverse") { state =>
  val extracted = Project.extract(state)

  val newState = extracted.append(Seq(name := name.value.reverse), state)

  Project.runTask(greet, newState)

  state // Reset the state back to what it was originally
}

lazy val root = (project in file("."))
  .settings(
    name := "world",
    commands ++= Seq(greetReverse),
    greet := {
      println("Hello " + name.value + "!")
    }
  )

/** sbt output:
  *
  * > greet
  * Hello world!
  * > greetReverse
  * Hello dlrow!
  * > greet
  * Hello world!
  */