Preheat the oven to 200°C To make the candied zest (for decorating), remove the orange zest using a vegetable peeler, taking care not to include any white pith. Cut into thin strips. Put 50g sugar into a pan with 150ml water and heat until dissolved. Add the zest and simmer for 10 minutes or until tender. Drain and leave to cool.
Put the rhubarb in a single layer in a shallow greased dish and sprinkle on the rest of the sugar. Add the juice from the orange, cover with foil, and cook in the oven for 15 minutes. Remove the foil, turn the pieces over, and cook, uncovered, for 10 minutes. Leave to cool. Drain and reserve the juice.
Split the sponges and sandwich with marmalade. Cut each into three pieces and put in the base of a trifle bowl or individual dishes. Mix 5tbsp rhubarb juice with the liqeur and sprinkle over the sponges. Put the rhubarb on top.
Mix the custard with the mascarpone and spoon over the rhubarb. Level the top and chill for 20 minutes.
Whip the cream until it just holds its shape and spread on top. Decorate the top with candied zest.