Bring coconut milk, lemongrass, ginger, and cilantro to a simmer in a small saucepan over medium-high heat. Remove from heat; let stand 1 hour.
Pour through a fine sieve into a medium bowl; discard solids. Whisk in egg yolks, salt, and pepper. Ladle into four 8-ounce, 2-inch-high ramekins. Divide crabmeat evenly among ramekins; sprinkle each with 3 or 4 cilantro leaves.
Fill a large pot fitted with a round wire rack or a steamer insert with enough water to just come up to rack. Bring water to a simmer over medium-high heat. Carefully place ramekins in steamer; cover pot. Steam custards over medium heat, checking occasionally to make sure water doesn't completely evaporate, until just set, 18 to 25 minutes. Place each custard on a saucer. Serve immediately with crisp toasts or crackers.