Poach the scallops. If you find a tough “foot” on the side of the scallops, pull it off. If the scallops are larger than ½ inch, cut them in pieces about that size. Bring a 2-quart pot of water to a boil and salt it liberally. Add the scallops, stir, let the water return to a boil, remove from the heat and let stand 2 or 3 minutes, until they are barely done—they should still be just a bit transparent in the middle. Drain and pat dry on paper towels.
Finish the tostadas. If you’re using avocado, cut around the pit, pull the two halves apart, remove the pit, scoop the flesh into a bowl and coarsely mash. If using cream cheese or goat cheese, mash in a small bowl to make it spreadable. Season avocado or cheese with a little lime juice and salt.
Spread a portion of avocado or cheese on each tostada and top with a portion of scallops. Stir up the salsa macha and drizzle a tablespoon or so of the solids and a little oil over each tostada. Serve with lime wedges for each person to squeeze on al gusto.