TestImplicits.scala

package org.scalatra.servlet

import javax.servlet.http.HttpSession

trait TestImplicits extends ServletApiImplicits {
    override implicit def enrichSession(session: HttpSession): TestSerializationSession =
      new TestSerializationSession(session)
}

TestSerializationSession.scala

package org.scalatra.servlet

import javax.servlet.http.HttpSession
import java.io.{OutputStream, ObjectOutputStream}

object NullOut extends OutputStream {
  def write(b: Int) {}
}

class TestSerializationSession(session: HttpSession) extends RichSession(session) {

  //val oos = new ObjectOutputStream(System.out)
  val oos = new ObjectOutputStream(NullOut)

  override def update(key: String, value: AnyRef) {
    oos.writeObject(value)
    super.update(key, value)
  }

}

Use with

class TestYourServlet extends YourServlet with TestImplicits

addServlet(classOf[TestYourServlet])