Bring the granulated sugar, honey, salt and 2 tablespoons water to a gentle simmer in a small saucepan over medium heat. Cook, stirring, until the sugar dissolves. Transfer 1 1/2 tablespoons of the syrup to a small bowl; set aside. Add the rhubarb to the remaining syrup; cook, stirring occasionally, until the rhubarb breaks down and the mixture thickens, 15 to 20 minutes. Let cool slightly, then transfer to a blender and puree until smooth. Transfer to a bowl and refrigerate until cold, at least 1 hour.
Beat the heavy cream, mascarpone and confectioners' sugar with a mixer on medium-high speed until stiff peaks form. Fold one-third of the cream mixture into the rhubarb mixture until just combined, then fold in the rest. Cover and refrigerate until cold, about 1 hour.
Meanwhile, preheat the oven to 375 degrees F. Use a 2-inch-round cookie cutter to cut circles from the pound cake slices. Transfer to a baking sheet and bake until lightly toasted, about 5 minutes. Remove from the oven and immediately brush the cake rounds with the reserved syrup; let cool completely on the baking sheet.
Spread a layer of strawberry preserves on the cake rounds. Transfer the rhubarb cream to a piping bag fitted with a large star tip. Pipe the cream onto the cakes.