Even if security is disabled you'll get a SecurityException with the following code:

class A {
  def context
  def p
  def A(ctx) {
    this.ctx = ctx
    this.p = [ "a": {this.a}]
  }
  def a() {
    ctx.echo "hi"
  }
  def b() {
    ctx.node("master") {
      ctx.parallel p
    }
  }
}

(new A(this)).b()

However flattening the parallel as

 ctx.parallel ["a": {this.a}]

is OK, as is making a local variable for the parallel mapping.  I believe this behavior changed with the recent update to Jenkins security.