Combine butter and next 8 ingredients in a large bowl; chill until ready to use.
Combine clams and 3/4 cup water in a large nonstick skillet over medium-high heat. Bring to a boil, cover, and simmer 5 minutes or until clams open. Discard clams that remain closed. Drain; return clams to pan. Add butter mixture and remaining 1/2 cup water to pan. Cook over medium-high heat 2 minutes or until butter melts.
Grill bread and lime slices 1 to 2 minutes on each side. Arrange bread on platter, top with clams and butter sauce, and serve with grilled lime slices. Garnish, if desired. Serve immediately.