Combine 1 cup water, fronds, and seeds in a small saucepan. Bring to a boil; remove from heat. Pour into a large bowl; cool to room temperature. Add remaining 2 1/2 cups water, salt, sugar, and liqueur, stirring until salt and sugar dissolve. Pour salt mixture into a large zip-top plastic bag. Add ice and pork; seal. Refrigerate 4 hours, turning bag occasionally.
Prepare grill to high heat.
Remove pork from bag; discard brine. Pat pork dry with paper towels. Coat pork with cooking spray. Combine the rosemary, pepper, and garlic; rub evenly over both sides of pork. Place pork on grill rack coated with cooking spray; grill 3 minutes on each side or until done. Let stand 5 minutes before serving.