JScript.NETのグローバル属性はC#と全く同じで
[assembly: 〜]で行けたのですね。

import System;
import System.Reflection;
[assembly: AssemblyTitle("AttributeTest1")]
[assembly: AssemblyCompany("Nullposoft")]
[assembly: AssemblyProduct("MyApplication1")]

public AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)
class Author extends Attribute
{
private var name : String;
public var version: double;

public function Author(name : String)
{
this.name = name;
version = 1.0;
}
}

public Author("H. Ackerman", version = 1.1)
class SampleClass
{
// H. Ackerman's code goes here...
}