is there a substitute for diastatic malt powder

You can use a small- very small - as a substitute. If you're having trouble finding barley malt powder, home brew stores are an excellent and inexpensive source.

The recipe at the ATK website requires registration, but I found it elsewhere:

http://www.kcet.org/living/food/the-public-kitchen/weekend-recipe-baguettes.html

The recipes says the the malt powder is optional, so if you don't have it, you don't need to find a substitute.