This worked, and really tastes like orange marmalade. Couple notes: shredding rather than chopping the melon rind gives a more customary marmalade texture. And it's important to shred or grate at least one of the oranges down into the white part of the rind, as the bitterness is what gives the mixture its characteristic marmalade flavor. Taste the mixture when it has cooked about 30 minutes--I felt it needed more lemon juice. After about 2 hours of cooking, the mixture still didn't sheet from a spoon but it looked much more "syrup" like so I called it done and canned it.
Canning directions: boil jars and lids for at least 2 minutes in boiling water. Retrieve with tongs. Immediately fill hot jars with marmalade. Wipe the threads and top clean. Cap and tighten the lid. Invert the jar for 2 minutes. Turn it right side up. (These directions were at one time USDA approved, but I think the lawyers got to them. I've never lost a batch of jam processed this way.)