How can I specify an unordered Execution Block using the TPL Dataflow Library?

There is no such block in the library, but you can easily create one yourself by combining an ActionBlock and a BufferBlock. Something like:

public static IPropagatorBlock<TInput, TOutput>
    CreateUnorderedTransformBlock<TInput, TOutput>(
    Func<TInput, TOutput> func, ExecutionDataflowBlockOptions options)
{
    var buffer = new BufferBlock<TOutput>(options);
    var action = new ActionBlock<TInput>(
        async input =>
        {
            var output = func(input);
            await buffer.SendAsync(output);
        }, options);

    action.Completion.ContinueWith(
        t =>
        {
            IDataflowBlock castedBuffer = buffer;

            if (t.IsFaulted)
            {
                castedBuffer.Fault(t.Exception);
            }
            else if (t.IsCanceled)
            {
                // do nothing: both blocks share options,
                // which means they also share CancellationToken
            }
            else
            {
                castedBuffer.Complete();
            }
        });

    return DataflowBlock.Encapsulate(action, buffer);
}

This way, once an item is processed by the ActionBlock, it's immediately moved to the BufferBlock, which means ordering is not maintained.

One issue with this code is that it doesn't observe the set BoundedCapacity well: in effect, the capacity of this block is twice the capacity set in options (because each of the two blocks has a separate capacity).