Are Thoroughbred x Dutch Draft crosses registerable?

No.

The DHH is a fancy harness breed. The majority of thoroughbreds offer speed, and drafts offer strength. In real life, this cross would not give a DHH, so we are not allowing it here.

We are also doing this to add more of a challenge

I know a Dutch/TB cross won't be registerable but If I have a Dutch Draft/TB Mare that gets bred to a Dutch Warmblood stallion would the baby be eligible for registration?

My bad, I thought I had added that to this post already.

But, no, in order to be eligible the horse must not have any dd x tb blood in its breeding pedigree. The main reason I decided on this is because of how advanced thoroughbreds are in comparison to dws. They would give an unfair advantage, and make this club less of a challenge. Also for the same reason that a horse with a decent percentage of dd x tb blood would not be very typey.