In a medium bowl, mix the beer, mustard, and caraway seed, if using. Add the beer mixture to the pan. Grill over Direct Medium heat for about 20 minutes, turning the bratwurst 2 or 3 times. Remove the bratwurst from the pan but leave the pan where it is to continue to cook the sauerkraut mixture. Grill the bratwurst over Direct Medium heat until nicely browned, 2 to 3 minutes, turning once. Serve the bratwurst warm in hot dog buns with the sauerkraut mixture. Serve more mustard on top, if desired.