It seems to me that the key ingredient in the mix is the Expandex® Modified Tapioca Starch. If you can eat tapioca, you should be able to substitute any of the other ingredients and get a suitable mix. It would take a little guess work to get the right proportions, but I think it could be done.
Heres a list of sample substitutions:
Expandex® Modified Tapioca Starch - no substitution
Potato Starch - substitute arrowroot or cornstarch
Corn Starch - substitute potato starch or arrowroot
Corn Flour - substitute quinoa or similar
White Rice Flour - difficult to substitute, but could try almond flour, buckwheat, quinoa, etc.
Xanthan Gum
The Expandex® Modified Tapioca Starch is expensive, but it is available for purchase. Here is a list of places to try:
http://expandexglutenfree.com/consumers ... to-buy.php
Note: I haven't tried this, so I can't recommend it. I'm not sure if I can eat tapioca. My point is that we shouldn't let intolerances stop us from creating our own GF flour blends. Most starches are interchangeable, and many different flours can usually be used.
Gloria
You never know what you can do until you have to do it.