Position oven racks in the center and top third of the oven and preheat to 350 F. Line two baking sheets with parchment paper or silicone baking mats.
Pulse the oats and 1/4 cup of the brown sugar in a food processor fitted with the metal chopping blade, until the oats are flourlike. Add the rice flour, cornstarch, cinnamon, baking soda, and salt, pulse to combine.
Using an electric mixer at medium-high speed, beat the butter and remaining 1 1/4 cups of brown sugar in a large bowl until the mixture is light in color and texture, about 3 minutes. Gradually beat in the eggs, beating well, then the vanilla. Briefly mix in the flour mixture until mixed. Add in the milk and 1 cup of the trail mix. Drop tablespoonfuls of the dough on the cookie sheets, spacing them 2 inches apart. Use the remaining ¼ cup trail mix to top the cookies with bits of fruit and nuts or chocolate chips for a nice garnish.
Bake, switching the positions of the sheets from top to bottom and front to back halfway through baking, until the edges of the cookies are lightly browned, 10 to 12 minutes. Cool on the sheets for a few minutes, then carefully transfer to a wire cake rack to cool completely.