In large bowl, whisk together brown, white and sweet rice flours, tapioca starch, granulated sugar, xanthan gum and salt. Using pastry blender or 2 knives, cut in butter until in coarse crumbs with a few larger pieces.
Stir ice water with vinegar; drizzle over flour mixture, tossing with fork until ragged dough forms and adding up to 2 tbsp more ice water if necessary. Divide in half; shape into discs. Wrap each and refrigerate until chilled, about 30 minutes. (Make- ahead: Refrigerate for up to 24 hours.)
Between 2 pieces of parchment paper, roll out 1 disc, pressing edges together if cracks form, into 12-inch (30 cm) circle. Peel off top paper; replace over pastry. Gently turn over; peel off top paper. Transfer, pastry side down, to 9-inch (23 cm) pie plate; peel off paper. Fit pastry into plate, trimming, if necessary, to leave 3/4-inch (2 cm) overhang.
Filling: In large bowl, toss together rhubarb, strawberries, sugar, tapioca starch, vanilla and ginger. Scrape into pastry shell; dot with butter.
Between 2 pieces of parchment paper, roll out remaining pastry disc into 9-inch (23 cm) circle. Peel off top paper; replace over pastry. Gently turn over; peel off top paper.
Whisk egg yolk with 2 tsp water; brush some over rim of pastry shell. Place pastry circle over filling; peel off paper. Trim, if necessary, to leave 3/4-inch (2 cm) overhang. Fold overhang under bottom rim; crimp with fork. Brush top with remaining egg mixture; sprinkle with coarse sugar. Cut steam vents in top.
Bake on bottom rack in 425F (220C) oven for 10 minutes. Reduce heat to 350F (180C); bake until bubbly and bottom is golden, about 1 hour. Let cool in pan on rack.