Line a microwave-safe muffin tin (such as silicone) with 2 paper liners per cup. In a large bowl, combine the bran with the milk and grated squash. Let stand for 5 minutes, stirring once more to make sure the bran is evenly wet. In a medium bowl, mix the dry ingredients. In a small bowl, whisk the orange juice concentrate, oil, lemon juice, and egg. Add the wet ingredients to the bran mixture, stir to incorporate, then add the dry ingredients, stirring just until combined. Do not overmix. Fill each cup half full. Microwave on high power for 5 to 5½ minutes if using a silicone pan, or for 2 to 3 minutes if using stand-alone paper cups. The muffins will be springy when touched. It is best to microwave only 3 muffins at a time.
Cooking Note: If you prefer to bake the muffins in a regular oven, line a standard muffin tin with cupcake papers and bake at 350°F for about 18 minutes, or until a skewer inserted in the center comes out clean.