Make the sauce by combining the zest, ginger, honey, cayenne, and salt in a food processor (or use a hand blender) and process until smooth. Add the lemon juice, rice vinegar, and soy suace, and pulse to combine. With the machine running, drizzle in the oils.
Slice the watermelon into cubes that are roughly 1/2-inch - 3/4-inch in width. Transfer to a large bowl or container. Pour the sauce over the top of the watermelon and gently toss. Refrigerate for a few hours, or overnight. If you remember to toss the watermelon once or twice along the way, even better.
Use a slotted spoon to transfer the watermelon poké to a serving bowl or platter, shaking off extra sauce. Reserve any remaining sauce to use as seasoning (for a poke bowl, or to season rice, etc.). Sprinkle with sliced green onions, and sesame seeds.