Gradually whisk hot lemon mixture into egg mixture. Stir until beginning to thicken and thermometer inserted into mexture registers 160 degrees F, about 4 minutes. Transfer curd to small bowl. Puree thawed boysenberries and 4 tablespoons sugar in blender. Using electric mixer, beat cream in large bowl until soft peaks form.
Spoon mousse into pastry bag fitted with large plain tip.
Repeat layering with mousse and curd. Top with reserved whipped cream. Garnish with fresh boysenberries, if desired. With a very sharp paring knife, or serrated tomato knife, slice off the top of the tomatoes and discard. Using a demitasse spoon, scoop out the seeds and discard.