Cover the bottom of a 9x13-inch pan with cake slices. Add the fruit juice to a saucepan and sprinkle the agar-agar over the juice; let soak for 15 minutes. Bring the juice to simmer and stir until the agar-agar has dissolved. Remove the pan from the heat and let sit until the juice starts to thicken. Stir 1 cup sliced strawberries and 1 cup blueberries into the slightly thickened juice. Pour the fruit and juice mixture over the pound cake slices and let sit until the juice has set.
In a large bowl, whip the heavy cream until foamy and peaks are just about to form. Beat in the honey until stiff peaks form. Spread the whipped cream over the cake. Arrange the blueberries on the whipped cream for the stars of the flag. Place the strawberry halves on the whipped cream to resemble the stripes of a flag.
Derived from seaweed, agar is a vegetarian alternative to gelatin. If you cannot locate agar, you can use unflavored gelatin. You will need 4 envelopes. Soften gelatin in 1 cup of the juice. Boil the remaining 3 cups and stir into the softened gelatin mixture. Gelatin will need to be refrigerated for each of the subsequent steps, as it does not set up at room temperature (as agar does).