How do I trap arguments to a target method when using a Proxy object?

There actually is a way to do this, of course! I just hadn’t thought it through thoroughly enough. I can just return a ‘proxy’ function and trap the arguments in there:

var test = {
    doSomething: function() {
        console.log( arguments.length );
    }
};

var testProxy = new Proxy( test, {
    get: function( target, property, receiver ) {

        switch( property ) {
            case 'doSomething':
              // you just have to return a proxy function
              return function() {
                  // arguments accessible, after all!
                  console.log( 'testProxy::doSomething() arguments.length: ' + arguments.length );

                  // here you can still invoke the original method, of course
                  target[ property ].apply( this, arguments );
              }
            break
        }

        return target[ property ];
    }
} );

testProxy.doSomething( 'this', 'is', 'not', 'so', 'lame', 'after', 'all' );