How can one clarify orange juice to clear orange flavoured gelatine?
You might try simple agar clarification.  Dave Arnold has written extensively on the technique.  Basically, you use agar to create an orange juice gel. Once set, you use a whisk to turn the gel in to curds.  Pour into a cloth, hang, and allow to drain. Done correctly, the orange juice will be clear.  You can google the technique.  It's fairly easy and doesn't take too long.  You can then manipulate that liquid in many ways.  This is not how marmalade is made, but it will allow you to clarify almost any fruit or vegetable based juice/puree without a centrifuge.