Transparent HEAD request handling in Play 2.1. Note that it may be more efficient to implement HEAD support in an action directly, as this may avoid unnecessary opening of resources or rendering things.

Global.scala

import play.api.libs.iteratee.{Done, Iteratee, Enumerator}
import play.api.mvc._
import play.api._
import play.api.libs.concurrent.Execution.Implicits._

object Global extends GlobalSettings {

  override def onRouteRequest(req: RequestHeader) = {
    // Lookup handler
    super.onRouteRequest(req) match {
      // If none was found and this is a HEAD request
      case None if req.method == "HEAD" => {

        // Try to look an action up again, but this time pretend it was a GET request
        super.onRouteRequest(req.copy(method = "GET")) match {
            
          // We found one, wrap it in a new action
          case Some(wrapped: EssentialAction) => Some(new EssentialAction() with RequestTaggingHandler {
            
            def apply(req: RequestHeader) = {
              
              def skipBody(result: Result): Result = result match {
                case SimpleResult(header, body) => {
                  // Tell the body enumerator it's done so that it can clean up resources
                  body(Done(()))
                  // Create a copy of the result, but with no content
                  SimpleResult(header, Enumerator(Results.EmptyContent()))
                }
                case ChunkedResult(header, body) => {
                  // Same as for SimpleResult
                  body(Done(()))
                  ChunkedResult(header, (it: Iteratee[Array[Byte], Unit]) => it.run)
                }
                // Unwrap
                case AsyncResult(future) => AsyncResult(future.map(skipBody))
              }
              
              // Invoke the wrapped action and skip the body it returns
              wrapped(req).map(skipBody)
            }

            // Ensure that request tags are added if necessary
            def tagRequest(request: RequestHeader) = wrapped match {
              case tagging: RequestTaggingHandler => tagging.tagRequest(request)
              case _ => request
            }
          })
          
          case other => other
        }
      }
      case other => other
    }
  }
}

Beware unintended consequences.

Although GET is safe and idempotent at the protocol level, there are other side-effects that may not be desired if the request was for HEAD.

I recommend making this something that can be explicitly turned on through configuration for those who want it as the default for all routes, and also reusable for those who may want this behaviour but only on selected routes.