In a shallow bowl, add ½ of the reserved pineapple juice and sprinkle an even layer of the gelatin across the surface of the juice. Allow to bloom for at least 5 minutes.
In the cold bowl of a stand mixer fitted with the whip attachment, beat the heavy cream on high speed until stiff peaks are formed, about 6-8 minutes. Transfer to an appropriate sized container and reserve in the refrigerator. Clean the stand mixer bowl and whip attachment and reattach. Combine the yolks and sugar together and beat on high until pale yellow, about 2 minutes. Heat the remaining pineapple juice and add very slowly to the egg mixture so as not to scramble the eggs. Heat the bloomed gelatin over a double boiler just enough to dissolve the sugar and add to the egg mixture. Fold in the reserved whipped cream.
Line the bottom of a lightly greased loaf pan with 3 of the pineapple rings. Cut 4 rings in half to form 8 semi-circles. Stand 3 of the semi-circles on each side and 1 on each end. Fill with the mousse from above. Refrigerate overnight. To remove, run the edge of a paring knife dipped in hot water around the edges, turn upside down and lightly tap the bottom of the pan to release the mousse. Slice with a hot knife and serve.