You could partially cook the corned beef the day before and refrigerate it. When you take it out of the fridge, the fat will have risen to the top and hardened, so you can remove it easily. (This works with any kind of meat or even chicken.)
Cook it for about 6 hours to get it tender as butter. The fat by then will have rendered out. Any left over fat can just be scraped off the meat very easily with a butter knife. Corned Beef is not marbled at all so the fat is in one area and the meat is in another.
You can get a corned beef round instead of brisket. That is quite lean and you cook the same way. I prefer it to brisket actually. It slices so nicely and evenly and makes great corned beef sandwiches.