In a Lexan or large shallow container, combine tequila, both liqueurs, hoja santa and tarragon tea, and spice mixture. Infuse for 2 hours, then strain out spices. Return liquid to container. In a jug or container with a spout, combine lime juice and grapefruit juice. Heat coconut milk to 170 degrees F, stirring constantly. Transfer hot coconut milk to a jug or container with a spout. Very slowly and simultaneously pour both coconut milk and citrus over entire surface of punch. Milk will break initially, then become cloudy. After 30 minutes, a layer of curds should form. Rest punch at room temperature for 1 hour, then transfer to a refrigerator for 1 hour. Strain punch through cheesecloth, then through coffee filters or a Superbag and refrigerate.
Pour approximately 3 oz into a rocks glass over a large ice cube.
Top with club soda.
*Hoja santa and tarragon tea: Bring 1 L filtered water to boil. Add 1 large hoja santa leaf, 5 tarragon sprigs and simmer for 20 minutes. Strain through fine-mesh strainer and chill in ice bath.
**Spice mixture: Toast 1 3/4 oz cinnamon sticks, 1 oz star anise pods, 1/3 oz Szechuan peppercorns, 1/3 oz whole cloves, 1 oz green cardamom pods and 1/3 oz fennel seeds in a pan over medium heat until fragrant for no more than 5 minutes. (Start with cinnamon and star anise, then add rest as they toast quickly.) Remove and let cool.