Step 1: Puree the pineapple in a food processor or blender. Pour it into a heavy nonreactive saucepan and bring to a boil over medium heat. Add the ginger marmalade and kejap manis. Let simmer until the marmalade melts, whisking to blend.
Step 2: Increase the heat to high and whisk in the rum. Let the sauce boil until thick and richly flavored, 3 to 5 minutes. Taste the sauce for seasoning, adding more kejap manis as necessary and salt and pepper to taste (you may not need much salt on account of the soy sauce). Let the sauce cool to room temperature before serving. It can be refrigerated, covered, for several weeks. Let the sauce return to room temperature before using.