Is it possible to turn old tomatoes into flour?
The recipe was most likely referring to Tomato Powder which is available from the Spice House in Milwaukee, WI.

I don't think you'd be able to produce the same result at home.  Even if the tomatoes could be dehydrated to the point where they are crisp enough to be ground, I don't think you could pulverize them fine enough very easily.  Even if it is possible, the trouble wouldn't outweigh simply purchasing the commercial product.