On their FAQ page, the Warm Company's answer to the question, "Is it safe to put Warm and Natural or Warm and White in the microwave?" is
"The Warm Company does not recommend cotton batting, polyester batting or blended batting for microwave use. Cotton batting, cotton fabric and cotton threads are flammable. Patterns recommending the use of Warm & Natural, Warm & White or Warm & Plush for microwave use are doing so without the consent of The Warm Company."
Pellon makes Wrap and Zap. Their website says
Pellon®’s Wrap-N-Zap® is a microwave safe product is made of 100% Natural Cotton Batting. When used in the microwave, it will trap the moisture inside to produce a perfect baked potato or a well steamed ear of corn. It can even be used for warming rolls and tortillas.
They say not to use it in the microwave without food.