Combine juniper berries and cardamom pods in a cheesecloth sachet. Heat Zinfandel, sugar, kumquats, 1/2 orange (sliced), 1/2 Meyer lemon (sliced), and spice sachet over medium heat until sugar dissolves, about 5 minutes.
Remove sachet and fruit; discard. Divide among 6 mugs or
heatproof glasses. Stir Grand Marnier or other orange-flavored liqueur into each. Garnish with orange or lemon slices.