Place the guava paste, orange juice, rum, and orange zest in a food processor with the plastic knife blade and pulse until pureed and spreadable.
Gently unroll the cooled cake. Evenly spread the guava mixture over the cake, leaving a 1-inch border around the edges uncovered. Carefully roll up the cake, using the towel to help you, and place it seam side down on a work surface. If necessary, use a sharp knife to trim off about ½ inch at each end of the roll to neaten the edges. Carefully transfer the jelly roll to a serving platter. Sprinkle lightly with the powdered sugar. Slice the cake and serve, dolloped with the whipped cream, if desired.