Put the rhubarb into a large ovenproof dish. I find a rectangular size measuring 12 by 8 by 2 inches perfect for the job. Sprinkle over the sugar, add the orange juice and zest, and 2 cups of water, and cover, either with lid or with foil. Bake for 1 hour. Take out of oven, remove lid, and let cool.
Strain carefully into a large measuring cup. I find this gives about 3 cups. Put the pulp aside (you can freeze it for use in the custard or trifle). If using granulated gelatin, soften it in 1/4 cup of the Muscat, about 5 minutes. Heat the mixture in the top of a double boiler over simmering water until the gelatin has dissolved, about 1 minute, and add to the rhubarb juice. Pour the remaining Muscat into the juice to bring it up to 3 3/4 cups; if not using granulated gelatin, add the Muscat as is to the juice to the required measurement. Taste; you may want some more sugar or a squeeze more orange or, indeed, more Muscat.
Lightly oil a 4-cup jelly mold by dabbing a paper towel in some suitably flavorless oil and then rubbing it over the interior of the mold. Soak gelatin leaves in a dish of cold water until softened. Put 2 ladles of rhubarb and Muscat syrup in a saucepan and bring to a boil. Squeeze out the gelatin leaves and whisk into the syrup. When they've dissolved, pour the contents of the pan back into the measuring cup. If you want to make sure everything's well enough blended, you can pour from the cup to the pan and back into the cup again. Pour into the jelly mold and place in the refrigerator to chill and set, about 6 hours or overnight.