Combine the sweet rice, almond, oat, and tapioca flours with sugar, baking powder, and salt in a large bowl. Whisk to combine. Add butter slices and lemon zest, and blend with a pastry cutter or fingertips until the butter resembles small peas. In a separate measuring glass, whisk together milk and egg. Gradually add the milk mixture to the flour mixture, mixing with a silicone spatula until the dough holds. Knead the dough into a ball, shape into a 6-inch disk, and wrap tightly. Chill until firm, at least 30 minutes or up to overnight. While chilling, rinse and dry the blueberries, cutting them in half if large.

Preheat the oven to 350ºF (180ºC) and position a rack in the upper third. Butter an 8-inch round or square pan and line with parchment paper. Roll the dough out into a 14x10 inch rectangle on parchment paper dusted with oat flour. Sprinkle blueberries and a bit of sugar evenly over the dough. Roll the dough into a log, ending seam-side down. Cut into 8-9 equal pieces and place the rounds into the prepared pan, cut-side up. Flatten slightly and brush tops with milk or cream. Sprinkle with extra sugar. Bake for 40-55 minutes until golden and cooked through. Cool for 20-30 minutes.

Invert a plate over the pan, flip to release biscuits, and remove the parchment. Flip again to right-side up. Whisk the powdered sugar, mascarpone, and lemon juice to create icing. Drizzle over warm biscuits and sprinkle with freeze-dried blueberries, lemon zest, or bee pollen. Serve warm or at room temperature. Store leftovers airtight for 1 day at room temperature or up to 3 days refrigerated. Re-warm before serving.