Providing workflow extensions to a workflow service - WF 4.0

Question

Greetings one and all!

I'm new to WF 4.0 and WWF in general so forgive me if this seems like a newbie type of question, but believe me I've scoured the depths of the Internet for a solution to this problem, but to no avail.

I have created a sample WF application with a custom CodeActivity that requires an extension be provided, as per below:

public sealed class PreparePizza : CodeActivity
{
    public InArgument<Order> Order { get; set; }

    protected override void CacheMetadata(CodeActivityMetadata metadata)
    {
        base.CacheMetadata(metadata);

        if (this.Order == null)
            metadata.AddValidationError("You must supply an Order.");

        metadata.RequireExtension<IPreparePizzaExtension>();
    }
    // If your activity returns a value, derive from CodeActivity<TResult>
    // and return the value from the Execute method.
    protected override void Execute(CodeActivityContext context)
    {
        // Obtain the runtime value of the Text input argument
        Order order = context.GetValue(this.Order);
        var extension = context.GetExtension<IPreparePizzaExtension>();
        extension.Prepare(order);
    }
}

public interface IPreparePizzaExtension
{
    void Prepare(Order order);
}

I then slot this activity into a workflow service and attempt to consume via my web app by adding a service reference. However, when I add the reference I get:

System.Activities.ValidationException: An extension of type 'PizzaMan.ActivityLibrary.IPreparePizzaExtension' must be configured in order to run this workflow.

Fair enough - of course my activity requires that I pass it an implementation of IPreparePizzaExtension - after all, I've told it to!

So my question is, how on earth do I pass this to the service? I can manage this easily enough in a console app scenario, using the WorkflowInvoker, but I cannot see any obvious way to do this via the service approach. I would assume that obviously a programmatic approach to adding the reference is what's needed, but again I'm at a loss as to precisely how to go about this.

Any help would be greatly appreciated.

Best regards Ian